docs(site): enhance extras content and add search, sitemap, and diagr… (

a2aproject#967)

docs(site): enhance extras content and add search, sitemap, and diagram plugins

- Expand Android HTTP client docs with usage guide, auto-discovery, and
  feature list
- Add Roq Lunr search plugin with overlay, button, and search index
  template
- Add Roq sitemap and diagram plugins
- Extract roq.version property to deduplicate hardcoded versions
- Fix HTML entities in markdown (&lt; → <, &ge; → >=)
- Fix search overlay z-index stacking and code copy button styling

Signed-off-by: Emmanuel Hugonnet <ehugonne@redhat.com>