[Docs][Content] Github install, UI changes, platform selection (#1098)

## Summary

This PR improves the documentation for GitHub authentication setup and
self-hosting.

## Changes

### GitHub OAuth/App Setup Guide
- Updated
[github.mdx](cci:7://file:///Users/madison/source/stack-auth/docs/content/docs/%28guides%29/concepts/auth-providers/github.mdx:0:0-0:0)
with clearer instructions differentiating between **GitHub OAuth App**
and **GitHub App** setup
- Added better explanations for when to use each option

### Self-Hosting Documentation
- Added prominent danger warning about self-hosting responsibilities
- Migrated inline shell commands to structured code examples using
[PlatformCodeblock](cci:1://file:///Users/madison/source/stack-auth/docs/src/components/mdx/platform-codeblock.tsx:242:0-673:1)
component
- Created
[docs/code-examples/self-host.ts](cci:7://file:///Users/madison/source/stack-auth/docs/code-examples/self-host.ts:0:0-0:0)
with all self-hosting commands

### Info Component
- Added new `danger` type for critical warnings with red accent styling
- Updated component styling with modern left accent bar and gradient
backgrounds

### PlatformCodeblock Component
- Added `hidePlatformSelector` prop to hide platform dropdown for
single-platform code examples
- Added Shell platform support for terminal commands (Docker, Git, pnpm)
- Filtered Shell platform from user-selectable options in both the
codeblock and header selectors

### Platform Config
- Added Shell platform with Docker, Git, and pnpm frameworks

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added comprehensive self-hosting and authentication customization
example collections for copy-paste use.
  * New "danger" info style with visual accent for important warnings.

* **Documentation**
* GitHub integration guide now centers on GitHub App with an alternate
OAuth path retained.
* Replaced many inline snippets with platform-driven code blocks and
improved platform/framework selector behavior (single-platform
optimization; option to hide selector).
  * Pages now surface "Last updated" above descriptions.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

docs/code-examples/customization.ts

@@ -0,0 +1,207 @@
+import { CodeExample } from '../lib/code-examples';
+
+export const customizationExamples = {
+  'sign-in': {
+    'default': [
+      {
+        language: 'JavaScript',
+        framework: 'Next.js',
+        code: `'use client';
+import { SignIn } from "@stackframe/stack";
+
+export default function DefaultSignIn() {
+  // optionally redirect to some other page if the user is already signed in
+  // const user = useUser();
+  // if (user) { redirect to some other page }
+  return <SignIn fullPage />;
+}`,
+        highlightLanguage: 'tsx',
+        filename: 'app/handler/sign-in/page.tsx',
+      },
+    ] as CodeExample[],
+
+    'custom-oauth': [
+      {
+        language: 'JavaScript',
+        framework: 'Next.js',
+        code: `'use client';
+import { useStackApp } from "@stackframe/stack";
+
+export default function CustomOAuthSignIn() {
+  const app = useStackApp();
+
+  return (
+    <div>
+      <h1>My Custom Sign In page</h1>
+      <button onClick={async () => {
+        // This will redirect to the OAuth provider's login page.
+        await app.signInWithOAuth('google');
+      }}>
+        Sign In with Google
+      </button>
+    </div>
+  );
+}`,
+        highlightLanguage: 'tsx',
+        filename: 'app/handler/sign-in/page.tsx',
+      },
+    ] as CodeExample[],
+
+    'custom-credential': [
+      {
+        language: 'JavaScript',
+        framework: 'Next.js',
+        code: `'use client';
+import { useStackApp } from "@stackframe/stack";
+import { useState } from "react";
+
+export default function CustomCredentialSignIn() {
+  const [email, setEmail] = useState('');
+  const [password, setPassword] = useState('');
+  const [error, setError] = useState('');
+  const app = useStackApp();
+
+  const onSubmit = async () => {
+    if (!password) {
+      setError('Please enter your password');
+      return;
+    }
+    // This will redirect to app.urls.afterSignIn if successful.
+    // You can customize the redirect URL in the StackServerApp constructor.
+    const result = await app.signInWithCredential({ email, password });
+    // It's better to handle each error code separately, but for simplicity,
+    // we'll just display the error message directly here.
+    if (result.status === 'error') {
+      setError(result.error.message);
+    }
+  };
+  
+  return (
+    <form onSubmit={(e) => { e.preventDefault(); onSubmit(); } }>
+      {error}
+      <input type='email' placeholder="[email protected]" value={email} onChange={(e) => setEmail(e.target.value)} />
+      <input type='password' placeholder="password" value={password} onChange={(e) => setPassword(e.target.value)} />
+      <button type='submit'>Sign In</button>
+    </form>
+  );
+}`,
+        highlightLanguage: 'tsx',
+        filename: 'app/handler/sign-in/page.tsx',
+      },
+    ] as CodeExample[],
+
+    'custom-magic-link': [
+      {
+        language: 'JavaScript',
+        framework: 'Next.js',
+        code: `'use client';
+
+import { useStackApp } from "@stackframe/stack";
+import { useState } from "react";
+
+export default function CustomMagicLinkSignIn() {
+  const [email, setEmail] = useState('');
+  const [error, setError] = useState('');
+  const [message, setMessage] = useState('');
+  const app = useStackApp();
+
+  const onSubmit = async () => {
+    if (!email) {
+      setError('Please enter your email address');
+      return;
+    }
+    
+    // This will send a magic link email to the user's email address.
+    // When they click the link, they will be redirected to your application.
+    const result = await app.sendMagicLinkEmail(email);
+    // It's better to handle each error code separately, but for simplicity,
+    // we'll just display the error message directly here.
+    if (result.status === 'error') {
+      setError(result.error.message);
+    } else {
+      setMessage('Magic link sent! Please check your email.');
+    }
+  };
+  
+  return (
+    <form onSubmit={(e) => { e.preventDefault(); onSubmit(); } }>
+      {error}
+      {message ? 
+        <div>{message}</div> :
+        <>
+          <input type='email' placeholder="[email protected]" value={email} onChange={(e) => setEmail(e.target.value)} />
+          <button type='submit'>Send Magic Link</button>
+        </>}
+    </form>
+  );
+}`,
+        highlightLanguage: 'tsx',
+        filename: 'app/handler/sign-in/page.tsx',
+      },
+    ] as CodeExample[],
+  },
+  
+  'sign-up': {
+    'default': [
+      {
+        language: 'JavaScript',
+        framework: 'Next.js',
+        code: `'use client';
+import { SignUp } from "@stackframe/stack";
+
+export default function DefaultSignUp() {
+  // optionally redirect to some other page if the user is already signed in
+  // const user = useUser();
+  // if (user) { redirect to some other page }
+  return <SignUp fullPage />;
+}`,
+        highlightLanguage: 'tsx',
+        filename: 'app/handler/sign-up/page.tsx',
+      },
+    ] as CodeExample[],
+
+    'custom-credential': [
+      {
+        language: 'JavaScript',
+        framework: 'Next.js',
+        code: `'use client';
+
+import { useStackApp } from "@stackframe/stack";
+import { useState } from "react";
+
+export default function CustomCredentialSignUp() {
+  const [email, setEmail] = useState('');
+  const [password, setPassword] = useState('');
+  const [error, setError] = useState('');
+  const app = useStackApp();
+
+  const onSubmit = async () => {
+    if (!password) {
+      setError('Please enter your password');
+      return;
+    }
+    // This will redirect to app.urls.afterSignUp if successful.
+    // You can customize the redirect URL in the StackServerApp constructor.
+    const result = await app.signUpWithCredential({ email, password });
+    // It's better to handle each error code separately, but for simplicity,
+    // we'll just display the error message directly here.
+    if (result.status === 'error') {
+      setError(result.error.message);
+    }
+  };
+  
+  return (
+    <form onSubmit={(e) => { e.preventDefault(); onSubmit(); } }>
+      {error}
+      <input type='email' placeholder="[email protected]" value={email} onChange={(e) => setEmail(e.target.value)} />
+      <input type='password' placeholder="password" value={password} onChange={(e) => setPassword(e.target.value)} />
+      <button type='submit'>Sign Up</button>
+    </form>
+  );
+}`,
+        highlightLanguage: 'tsx',
+        filename: 'app/handler/sign-up/page.tsx',
+      },
+    ] as CodeExample[],
+  },
+};