feat(site): add Docs & Languages links to the docs header (#377)

Bring the Starlight docs header to parity with the landing nav — now
Docs · Languages · GitHub · star pill, in that order. Added via the
SocialIcons slot (no full header rebuild, so search/theme/mobile keep
working); the text links and star pill are hidden on mobile, where the
sidebar already covers navigation.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>