docs: make the docs site theme usable on mobile and narrow screens

[Xuanwo]

Why

The custom "Lance Docs" theme introduced in #7821 was designed desktop-first: on phones and narrow windows the header overflows horizontally (the section tabs get squeezed to zero width while the non-shrinking action buttons push the page wider than the viewport), and the side navigation renders as a long static block above every article, forcing readers to scroll past the whole section index before reaching content.

This makes the theme responsive:

• Below 960px the header folds into two rows: brand plus icon-only actions on top ("Get started", the GitHub label, and the star count collapse away), with a horizontally scrollable section-tab row underneath.
• The side navigation collapses behind a disclosure button labeled with the current section, so articles lead on mobile.
• Anchor jumps now land clear of the sticky header (scroll-margin-top), the search overlay becomes an edge-to-edge sheet on small screens, and article headings, the footer, and the 404 page scale down.

Verified in a real browser at 390px and 320px widths (home, docs pages, tables, code blocks, dark mode, search) and regression-checked the desktop layout, which is unchanged.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between e934cc2 and 2999c28.

📒 Files selected for processing (5)

• docs/theme/assets/site.css
• docs/theme/assets/site.js
• docs/theme/base.html
• docs/theme/home.html
• docs/theme/main.html

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📝 Walkthrough

Walkthrough

The documentation theme adds a responsive sidenav toggle, updates header and sidebar markup, adjusts folded-layout behavior, and refines hero, heading-anchor, and mobile search styling.

Changes

Responsive documentation theme

Layer / File(s)	Summary
Sidenav disclosure wiring docs/theme/main.html, docs/theme/assets/site.js, docs/theme/assets/site.css	Adds an accessible menu button, synchronizes its expanded state with the sidenav’s open class, and styles the collapsed responsive navigation panel.
Responsive theme layout details docs/theme/home.html, docs/theme/base.html, docs/theme/assets/site.css	Structures hero and GitHub labels for responsive styling, adjusts heading scroll offsets, folds header controls below 960px, and makes mobile search edge-to-edge below 700px.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Reader
  participant SidenavToggle
  participant SiteJS
  participant Sidenav
  Reader->>SidenavToggle: Click menu button
  SidenavToggle->>SiteJS: Invoke click handler
  SiteJS->>Sidenav: Toggle open class
  SiteJS->>SidenavToggle: Update aria-expanded

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Possibly related PRs

• lance-format/lance#7821: Related Lance MkDocs theme refinements covering responsive sidenav behavior and documentation theme markup.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: making the docs site theme usable on mobile and narrow screens.
Description check	✅ Passed	The description matches the changeset and explains the responsive docs theme updates in relevant detail.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch xuanwo/docs-mobile-responsive-de9db4

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[prrao87]

Tested on a small local window, looks good. Thanks for the fix!