docs: add API documentation for Coder plugin

plugins/backstage-plugin-coder/README.md

@@ -89,7 +89,7 @@ the devcontainer.
    );
    ```
 
-   <!-- > Note: You can also wrap a single page or component with `CoderProvider` if you only need Coder in a specific part of your app. See our [API reference](#TODO) for more details. -->
+   **Note:** You can also wrap a single page or component with `CoderProvider` if you only need Coder in a specific part of your app. See our [API reference](./docs/README.md) (particularly the section on [the `CoderProvider` component](./docs/components.md#coderprovider)) for more details.
 
 1. Add the `CoderWorkspacesCard` card to the entity page in your app:
 

plugins/backstage-plugin-coder/docs/README.md

@@ -0,0 +1,9 @@
+# Plugin API Reference – Coder for Backstage
+
+For users who need more information about how to extend and modify the Coder plugin. For general setup, please see our main [README](../README.md).
+
+## Documentation directory
+
+- [Components](./components.md)
+- [Custom React hooks](./hooks.md)
+- [Important types](./types.md)