docs: add FAST Element 3.x API reference landing page

sites/website/src/docs/3.x/api.md

@@ -1,10 +1,75 @@
 ---
+id: api-reference
+title: API Reference
 layout: 3x
 eleventyNavigation:
   key: api3x
   title: API Reference
   order: 4
 navigationOptions:
   activeKey: api3x
-permalink: false
+permalink: /docs/3.x/api/overview/
+description: FAST Element 3.x API reference and package export guide.
+keywords:
+  - API
+  - reference
+  - exports
+  - imports
+  - stability
 ---
+
+# API Reference
+
+Use this page to choose the right FAST Element import path and then browse the
+generated API reference for the public surface of each entrypoint.
+
+## Root vs. path exports
+
+The root package export, `@microsoft/fast-element`, is the primary entrypoint for
+FAST Element's component authoring APIs. It is the best default when you are
+building custom elements with `FASTElement`, `html`, `css`, `attr`, directives,
+observables, state helpers, and other core primitives.
+
+FAST Element 3.x also publishes focused path exports. Use a path export when you
+want a smaller, feature-specific entrypoint or when the API is path-only. The
+path-only entrypoints should always be imported from their documented package
+paths rather than from the package root.
+
+## Common import choices
+
+| Need | Import from | Reference |
+|---|---|---|
+| Core component authoring APIs | `@microsoft/fast-element` | [Root API reference](/docs/3.x/api/fast-element/) |
+| Declarative HTML templates | `@microsoft/fast-element/declarative.js` | [Declarative API reference](/docs/3.x/api/fast-element/declarative/) and [Declarative HTML overview](/docs/3.x/declarative-templates/overview/) |
+| Server-rendered hydration | `@microsoft/fast-element/hydration.js` | [Hydration flow](/docs/3.x/declarative-templates/server-rendering/#hydration-flow) and [path export list](/docs/3.x/advanced/path-exports/) |
+| W3C Context protocol helpers | `@microsoft/fast-element/context.js` | [Context API reference](/docs/3.x/api/fast-element/context/) |
+| Dependency injection container APIs | `@microsoft/fast-element/di.js` | [DI API reference](/docs/3.x/api/fast-element/di/) and [Dependency Injection guide](/docs/3.x/advanced/dependency-injection/) |
+| Individual core features, such as `html`, `css`, `attr`, or directives | The matching path export, such as `@microsoft/fast-element/html.js` | [Path exports](/docs/3.x/advanced/path-exports/) |
+
+## Stability tags
+
+The generated API pages include release-tag callouts from API Extractor:
+
+- **Public** APIs are the supported surface for production code.
+- **Beta** APIs are preview APIs that can change based on feedback.
+- **Alpha** APIs are early previews and should be treated as less stable than
+  beta APIs.
+- **Internal** APIs are not intended for application code, even when they appear
+  in a type signature or generated page.
+
+Prefer public APIs when possible. If a generated page includes a beta or alpha
+notice, review the callout before depending on that symbol in production code.
+
+## Generated API docs
+
+The API reference pages are generated from FAST Element's API Extractor output
+and should not be edited by hand. Start with the generated
+[`@microsoft/fast-element` root reference](/docs/3.x/api/fast-element/) for the
+main package surface, then use the path-export references for focused APIs:
+
+- [`@microsoft/fast-element/declarative.js`](/docs/3.x/api/fast-element/declarative/)
+- [`@microsoft/fast-element/context.js`](/docs/3.x/api/fast-element/context/)
+- [`@microsoft/fast-element/di.js`](/docs/3.x/api/fast-element/di/)
+
+For the complete list of supported package paths, see the
+[Path Exports](/docs/3.x/advanced/path-exports/) page.