[codex] Fix search index generation

[miharp]

Summary

Add a repo-level search.json so local site search indexes the actual documentation pages instead of only the homepage.

Root cause

jekyll-vitepress-theme generates /search.json from site.data.sidebar, but this repo does not define _data/sidebar.*.

This docs site uses _data/nav/*.yml and collection defaults in _config.yml, so the theme's generated search index never iterates the documentation collections.

What changed

• Added a custom root search.json
• Indexed the homepage, regular titled pages, and output collection documents from site.documents
• Overrode the theme's broken default index generation for this repo structure

Validation

• Ran bundle exec jekyll build
• Verified _site/search.json contains 881 entries after the change
• Verified OpenVox docs pages are present in the generated index

Closes #80

[tuxmea]

That's great. I stumpled upon search not working by today, too.
Please undraft so I can approve. We can always improve later