feat(docs): add SubNav, DocItem layout, TOC, and Footer

[sl33ty]

Summary

• Adds a docs sub-navigation bar below the main navbar with tabs for each docs instance
• Custom BackToTop button and EndBar component added to the doc page layout
• TOC override for correct sticky behaviour with the double-height navbar
• Footer updated with dark mode support and theme toggle
• custom.css updated for sticky sidebar/TOC layout, 118px navbar height, dark mode, and mobile responsiveness

Notes

Depends on #121 (multi-instance docs setup) — the SubNav tabs link to routes defined there.

Test plan

• SubNav tabs render and navigate correctly
• Sidebar and TOC remain sticky while scrolling on long pages
• BackToTop button appears/disappears correctly
• Dark mode toggle works in footer and across docs pages
• Layout is correct on mobile

[sl33ty]

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