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

[janechu]

Pull Request

📖 Description

Adds a FAST Element 3.x API reference landing page that helps readers choose between the root package export and path-only entrypoints. The page explains stability tags, generated API docs, common import choices, and links to declarative, hydration, context, and DI references or guides.

📑 Test Plan

• npm run format:check
• npx biome format --no-errors-on-unmatched sites\website\src\docs\3.x\api.md
• npm run checkchange
• npm run build -w @microsoft/fast-site

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.