[WEB-4801] Initial docs rewrite design feedback fixes

[jamiehenson]

Human preamble

Addresses Jamie W's feedback here, plus subsequent feedback from Mark and Jamie in DMs.

To test the intro content in PageHeader, go here, it's the only page with the intro frontmatter populated.

Summary

This PR addresses design feedback and fixes for the docs redesign, implementing various UI/UX improvements and refinements to the core navigation components.

Changes

Inkeep Integration

• Changed default mode to "Ask AI" (chat mode) instead of search
• Removed bottom-right Inkeep button

Left Sidebar Improvements

• Fixed item highlighting behavior to properly respect accordion open/active page state
• Restored pre-expandable left sidebar section functionality
• Fixed accordion animation to work for both expansion and contraction

UI/UX Fixes

• Made all breadcrumb chevrons use neutral-900 color consistently
• Reintroduced API reference content into LeftSidebar
• Capped max width of inner container to 1280px for better content layout
• Reduced right sidebar highlighter width to 1px for better visibility
• Fixed LanguageSelector to only show arrow and dropdown when >1 language available

Content & Configuration

• Removed Article Footer unit
• Updated PageHeader to source description from intro field in frontmatter
• Added temporary intro frontmatter to chat/rooms/messages page

Analytics

• Added event tracking for header button clicks (Ask AI, Help Resources, CLI, Logout)

Code Cleanup

• Removed breadcrumbs console.log
• Cleaned up unused Inkeep configuration code

Test plan

• Verify header button click tracking is working (Ask AI, Help Resources, CLI, Logout)
• Test left sidebar accordion animations work smoothly in both directions
• Test left sidebar highlighting behavior with nested items and accordion states
• Verify left sidebar pre-expandable section functionality works
• Check content max-width constraint on large screens (1280px)
• Verify right sidebar active section indicator is visible (1px width)
• Verify breadcrumbs display correctly and all chevrons are neutral-900
• Verify language selector arrow only appears when multiple languages available
• Test Inkeep opens in chat ("Ask AI") mode by default
• Verify bottom-right Inkeep button is removed
• Verify API reference content displays in left sidebar
• Test PageHeader displays intro frontmatter content
• Confirm Article Footer is removed

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch web-4801-misc-docs-tweaks

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[jamiehenson]

temporary - here to show intro off - will be removed.