docs: add OAuth authentication documentation for AdminForth

Author: SerVitasik

adminforth/documentation/docs/tutorial/05-Plugins/11-oauth.md

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+# OAuth Authentication
+
+The OAuth plugin enables OAuth2-based authentication in AdminForth, allowing users to sign in using their Google, GitHub, or other OAuth2 provider accounts.
+
+## Installation
+
+To install the plugin:
+
+```bash
+npm install @adminforth/oauth --save
+npm install @adminforth/google-oauth-adapter --save  # for Google OAuth
+```
+
+## Configuration
+
+### 1. OAuth Provider Setup
+
+You need to get the client ID and client secret from your OAuth2 provider.
+
+For Google:
+1. Go to the [Google Cloud Console](https://console.cloud.google.com)
+2. Create a new project or select an existing one
+3. Go to `APIs & Services` → `Credentials`
+4. Create credentials for OAuth 2.0 client IDs
+5. Select application type: "Web application"
+6. Add your application's name and redirect URI
+7. In "Authorized redirect URIs", add next URI: `https://your-domain/oauth/callback`, `http://localhost:3500/oauth/callback`. Please remember to include BASE_URL in the URI if you are using it in project e.g. `https://your-domain/base/oauth/callback` 
+8. Add the credentials to your `.env` file:
+
+```bash
+GOOGLE_OAUTH_CLIENT_ID=your_google_client_id
+GOOGLE_OAUTH_CLIENT_SECRET=your_google_client_secret
+```
+
+### 2. Plugin Configuration
+
+Configure the plugin in your user resource file:
+
+```typescript title="./resources/adminuser.ts"
+import OAuthPlugin from '@adminforth/oauth';
+import AdminForthAdapterGoogleOauth2 from '@adminforth/google-oauth-adapter';
+
+// ... existing resource configuration ...
+
+plugins: [
+  new OAuthPlugin({
+    adapters: [
+      new AdminForthAdapterGoogleOauth2({
+        clientID: process.env.GOOGLE_OAUTH_CLIENT_ID,
+        clientSecret: process.env.GOOGLE_OAUTH_CLIENT_SECRET,
+      }),
+    ],
+    emailField: 'email',  // Required: field that stores the user's email
+  }),
+]
+```
+
+### 3. Email Confirmation
+
+The plugin supports automatic email confirmation for OAuth users. To enable this:
+
+1. Add the `email_confirmed` field to your database schema:
+
+```prisma title='./schema.prisma'
+model adminuser {
+  // ... existing fields ...
+  email_confirmed Boolean @default(false)
+}
+```
+
+2. Run the migration:
+
+```bash
+npx prisma migrate dev --name add-email-confirmed-to-adminuser
+```
+
+3. Configure the plugin with `emailConfirmedField`:
+
+```typescript title="./resources/adminuser.ts"
+new OAuth2Plugin({
+  // ... adapters configuration ...
+  emailField: 'email',
+  emailConfirmedField: 'email_confirmed'  // Enable email confirmation tracking
+}),
+```
+
+When using OAuth:
+- New users will have their email automatically confirmed (`email_confirmed = true`)
+- Existing users will have their email marked as confirmed upon successful OAuth login
+- The `email_confirmed` field must be a boolean type
+
+### 4. Open Signup
+
+By default, users must exist in the system before they can log in with OAuth. You can enable automatic user creation for new OAuth users with the `openSignup` option:
+
+```typescript title="./resources/adminuser.ts"
+new OAuth2Plugin({
+  // ... adapters configuration ...
+  openSignup: {
+    enabled: true,
+    defaultFieldValues: { // Set default values for new users
+      role: 'user',
+    },
+  },
+}),
+```
+
+### 5. UI Customization
+
+You can customize the UI of the OAuth login buttons by using the `iconOnly` and `pill` options.
+
+```typescript title="./resources/adminuser.ts"
+new OAuth2Plugin({
+  // ... adapters configuration ...
+  iconOnly: true, // Show only provider icons without text
+  pill: true, // Use pill-shaped buttons instead of rectangular
+}),
+```
+
+
+
+