[Docs][Content] - Rework overview page and slight updates to codeblocks (#1056)

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## Changes
### overview Page Redesign
- Added centered hero section with Stack Auth logo and welcome message
- Replaced verbose "Why Stack Auth" section with a clean Quick start
guide for next.js
- added Components section with compact card links. 
- Streamlined QuickLinks and Apps sections
- Added lastModified frontmatter support

### Frontmatter schema extension
- Extended frontmatter schema to support optional lastModified field
- Enables pages to display when they were last updated

### Last Modified Display
- Added subtle "Last Updated" text at the bottom of pages that have the
lastModified frontmatter field

### Copy Button for Code Blocks
- Added copy-to-clipboard button to  all codeblocks
- Button appears on hover, shows checkmark feedback when copied
- Applies to all code blocks

### New Code Examples
- Added setup/overview examples for the overview page quick start
	- install - npx command
	- use-auth - useUser example




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

* **New Features**
  * Code blocks gain a copy-to-clipboard button.
  * Docs pages now show "Last updated" timestamps when available.

* **Documentation**
* Overview page redesigned into a hero + quick-start, component
showcase, and explore sections with clearer links.
* Setup examples expanded to include an "install" quick-start and an
authentication "use" example.

<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/setup.ts

@@ -1,6 +1,27 @@
 import { CodeExample } from '../lib/code-examples';
 
 export const setupExamples = {
+  'overview': {
+    'install': [
+      {
+        language: 'JavaScript',
+        framework: 'Next.js',
+        code: `npx @stackframe/init-stack@latest`,
+        highlightLanguage: 'bash',
+        filename: 'Terminal'
+      }
+    ] as CodeExample[],
+    'use-auth': [
+      {
+        language: 'JavaScript',
+        framework: 'Next.js',
+        code: `const user = useUser({ or: "redirect" });
+return <div>Hi, {user.displayName}</div>;`,
+        highlightLanguage: 'tsx',
+        filename: 'page.tsx'
+      }
+    ] as CodeExample[],
+  },
   'setup': {
     'env-wizard': [
       {