Remove complex metadata file management and redundant steps.

Changes:
- Remove separate pages-metadata deployment
- Remove github-script step for CNAME cleanup
- Include .nojekyll and _config.yml directly in dist folder
- Simplify to single PR preview deployment step

This streamlines the workflow while maintaining all necessary
functionality for both main site and PR preview deployments.

Temporarily allow deployment from topic/gh-pr-preview branch
to restore the main site while testing PR previews.

This will be reverted before merging to main.

Remove topic branch from main site deployment condition.
Main site will only deploy from main branch going forward.

- Rename index.mdoc to index.mdx
- Convert Markdoc syntax ({% %}) to MDX JSX syntax
- Import components from @astrojs/starlight/components
- Maintain all existing functionality and content

Use import.meta.env.BASE_URL to properly construct links
so they work in both main site (/) and PR previews (/pr-preview/pr-N/).

Replace HTML button (which doesn't render) with clean markdown:
- Use heading with large link for preview
- Add table for metadata display
- Use blockquote for the note
- Update cleanup message to match style

- Use 👀 emoji for preview link (better than 🔗)
- Change workflow emoji from 🔄 to ⚙️ for better visual