Added App documentation (#3)

* added app documentation

* fixed link

docs/docs/03-api-documentation/01-app.md

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 # App
+
+## `StackServerApp` and `StackClientApp`
+
+`StackClientApp` provides all the functionality you can use to build a client-side application with Stack, like `signInWithOAuth`, `useUser`, `useProject`, etc. You can get a `StackClientApp` object by calling `useStackApp()` in a Client Component.
+
+`StackServerApp` is a subclass of `StackClientApp` that provides additional functionality for server side, like `getUser`, `getProject`, etc. To use your server app, you can just import the object from the file where you created it.
+
+## Initialization
+
+We showed in the [setup guide](/docs/getting-started/setup) that you can create a `StackServerApp` in a file like `lib/stack.ts` like this:
+
+```tsx
+import "server-only";
+import { StackServerApp } from "@stackframe/stack";
+
+export const stackApp = new StackServerApp({
+  tokenStore: "nextjs-cookie", // storing auth tokens in cookies
+});
+```
+
+Except for `tokenStore`, there are also a few other options you can pass to the `StackServerApp` constructor:
+
+### `urls`
+
+An object that contains some pre-defined URLs that Stack uses to route and redirect. You can override them for your own needs.
+
+Example:
+
+```tsx
+export const stackApp = new StackServerApp({
+  tokenStore: "nextjs-cookie",
+  urls: {
+    signIn: "/custom-sign-in",
+    signUp: "/custom-sign-up",
+    afterSignIn: "/user-profile",
+    afterSignUp: "/onboarding",
+  },
+});
+```
+
+
+- `home`  
+    Default: `"/"`  
+
+    The URL of the home page.
+
+-  `signIn`   
+    Default: `"/handler/sign-in"`  
+
+    The URL of the sign-in page.
+
+- `afterSignIn`    
+    Default: `"/"`
+
+    The URL that the user will be redirected to after successful signing in. Note the OAuth sign-in will distiguish if a user is registerd before or not, and not based on from where the user is clicking the OAuth sign-in button.
+
+- `signUp`    
+    Default: `"/handler/sign-up"`  
+
+    The URL of the sign-up page.
+
+- `afterSignUp`   
+    Default `"/"`
+
+    The URL that the user will be redirected to after successful signing up. Note the OAuth sign-up will distiguish if a user is registerd before or not, and not based on from where the user is clicking the OAuth sign-up button.
+
+- `signOut`   
+    Default: `"/handler/sign-out"`  
+
+    The URL of the sign-out page.
+
+- `afterSignOut`   
+    Default: `"/"`  
+
+    The URL that the user will be redirected to after successful signing out.
+
+- `emailVerification`   
+    Default: `"/handler/email-verification"`
+
+    The URL of the email verification page.
+
+- `passwordReset`   
+    Default `"/handler/password-reset"`
+
+    The URL of the password reset page.
+
+- `forgotPassword`   
+    Default `"/handler/forgot-password"`
+
+    The URL of the forgot password page.
+
+- `handler`    
+    Default: `"/handler"`  
+
+    The URL of the handler root. If you want to change the URL of your handler to something other than `/handler`, you can change it here. 
+
+    Note that you also need to change the `[...stack]/page.tsx` file to the corresponding location. When you want to use your website in a non-local environment, you need to change the handler URL in the Stackframe dashboard as well.