Add API reference overview (#7502)

* Add API reference overview

* Lint fix

Author: willeastcott

src/index.js

@@ -1,9 +1,45 @@
 /**
- * This module provides the core functionality for the PlayCanvas Engine. It includes the main
- * classes and methods used to create and manage a PlayCanvas application. It provides APIs for
- * graphics, audio, input, physics, asset management, scripting and much more. It also includes an
- * application framework and entity-component system, making it easy to manage the lifetime of your
- * application.
+ * Welcome to the PlayCanvas Engine API Reference. The PlayCanvas Engine is an open source framework
+ * for building interactive 3D applications. It is written in JavaScript and is built on top of
+ * standard browser APIs including WebGL, WebGPU, Web Audio and WebXR.
+ *
+ * ### 🧑 Who Should Use This Manual?
+ *
+ * This API reference is intended for developers who are building applications using:
+ *
+ * - The [PlayCanvas Engine](https://github.com/playcanvas/engine) directly.
+ * - [PlayCanvas Web Components](https://github.com/playcanvas/web-components) or
+ * [PlayCanvas React](https://github.com/playcanvas/react) which wrap the PlayCanvas Engine
+ * with a declarative interface.
+ * - The [PlayCanvas Editor](https://playcanvas.com/products/editor) which supports the writing of
+ * custom scripts based on the Engine API.
+ *
+ * ### 🔍 Searching the API Reference
+ *
+ * You can search the API Reference by clicking the 🔍 icon in the header. The `/` key is a shortcut
+ * for opening the search dialog. Press `Escape` to close the search dialog.
+ *
+ * ### 🗺️ Navigating the API Reference
+ *
+ * This top level page groups the API into a number of categories: Animation, Asset, Debug, Exporter,
+ * Gizmo, Graphics, Input, Math, Other, Physics, Script, Sound, User Interface and XR.
+ *
+ * There is also a full alphabetical index available on the left hand side of the page. However,
+ * this list is extensive so here is a list of key classes that you can use as jumping on points
+ * for exploring the API:
+ *
+ * - {@link AppBase} - Represents your PlayCanvas application.
+ * - {@link Scene} - Represents the graphical scene managed by the application.
+ * - {@link Entity} - Represents objects in your app. The scene manages a hierarchy
+ * of entities. Add capabilities to entities with {@link Component}s.
+ * - {@link ScriptComponent} - A powerful component that allows you to write {@link Script}s
+ * that implement custom behavior for your entities.
+ * - {@link AssetRegistry} - Manages all the {@link Asset}s (3D models, sounds, etc) in your app.
+ *
+ * ### 🙌 Contributing to the API Reference
+ *
+ * We want the API Reference to be as high quality as possible. If you spot any errors or omissions,
+ * please raise an issue or open a pull request on the [PlayCanvas Engine GitHub repository](https://github.com/playcanvas/engine).
  *
  * @module Engine
  */