Scope Algolia DocSearch to current docs platform and version

[simonhamp]

Summary

• When browsing docs (e.g. /docs/mobile/3/...), Algolia DocSearch results are now filtered to only show pages from the current platform and version using transformItems.
• Added docsearch:platform and docsearch:version meta tags to docs pages for future Algolia crawler facet support.
• Fixed Cmd+K opening duplicate DocSearch modals by initializing only one DocSearch instance and cloning the button into the mobile container as a proxy.

Test plan

• Feature tests for meta tag rendering on mobile, desktop, and non-docs pages
• Verify Cmd+K opens a single search modal
• Verify search results on /docs/mobile/3/... only show mobile v3 pages
• Verify search results on /docs/desktop/2/... only show desktop v2 pages
• Verify search on non-docs pages (e.g. /) returns all results
• Verify mobile menu search button still opens the search modal

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