docs: extend the API reference main doc

Author: pkosiec

docs/docs/api/index.md

@@ -4,5 +4,19 @@ This section contains the API reference for the AppKit packages.
 
 ## Modules
 
-- [`appkit`](appkit/index.md) - Core library
-- [`appkit-ui`](appkit-ui/index.md) - UI components library
+- [`appkit`](appkit/index.md) - Core library. Provides the core functionality for building Databricks applications.
+- [`appkit-ui`](appkit-ui/index.md) - UI components library. Provides a set of UI primitives for building Databricks apps in [React](https://react.dev/).
+
+## Getting started
+
+To learn how to get started with AppKit, see the [getting started guide](../index.md) for AI-assisted and manual quick start options.
+
+## Installation
+
+To install the AppKit packages into your existing JavaScript/TypeScript project, use `npm` or your package manager of choice:
+
+```bash
+npm install @databricks/appkit
+npm install @databricks/appkit-ui
+```
+