docs: add plugin manual and update sidebar

docs/manual/settings/plugins.md

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+# Plugin Manager
+
+Mivo v1.2+ introduces a powerful **Plugin System** that allows you to extend the core functionality of the application without modifying the source code.
+
+## Installing Plugins
+
+### Method 1: Upload ZIP
+If you have a plugin ZIP file (e.g. from a third-party developer), you can install it manually:
+
+1. Go to **Settings > Plugins**.
+2. Click the **Upload Plugin** button.
+3. Select your `.zip` file.
+4. Mivo will verify the structure (`plugin.php`) and install it.
+
+### Method 2: Via Registry
+You can browse the [Official Registry](/plugins/) to find community-verified plugins:
+
+1. find the plugin you want in the **Plugins** menu.
+2. Click **Download** to get the ZIP file.
+3. Upload it via **Settings > Plugins**.
+
+*(Future updates will allow one-click install directly from the dashboard)*
+
+## Managing Plugins
+
+In the **Settings > Plugins** table, you can see all installed extensions.
+
+- **Status**: Shows if the plugin is active.
+- **Actions**:
+  - **View Config**: Some plugins provide a configuration page.
+  - **Delete**: Removes the plugin and its files permanently.
+
+## Creating Plugins
+Developers can create custom plugins to add features like new Hotspot themes, payment gateways, or system tools. Check the [Contribution Guide](/docs/guide/contribution) for details.