docs: improve markdown docs script

[reidbarber]

llms.txt fixes:
- Add llms.txt for Internationalized
- Add root llms.txt
- Added descriptions next to links
- Fixed links to be *.md instead of *.mdx

Markdown fixes:
- Add custom rendering for LinkButton, VersionBadge, StaticTable, GroupedPropTable, ClassAPI, InterfaceType
- Include root-level pages (index.md)
- Use unordered list for listing icons and illustrations

I've identified a few remaining gaps in the markdown output, but we can the rest later.

MCP fix:
- Include page descriptions in initial index build (now that they’re in llms.txt)

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

For llms.txt and markdown fixes, check the built files.

To verify MCP fix, you'll need to build @react-spectrum/mcp locally and and test it with localhost:

yarn workspace @react-spectrum/s2-docs generate:md
yarn workspace @react-spectrum/mcp build
yarn start:s2-docs

{
  "mcpServers": {
    "React Spectrum": {
      "command": "node",
      "args": [".../react-spectrum/packages/dev/mcp/dist/index.js", "s2"],
      "env": {
        "DOCS_CDN_BASE": "http://localhost:1234"
      }
    }
}

🧢 Your Project: