docs: add site-wide sidebar, homepage GitHub link, and fix mobile table overflow

[uhyo]

Improves navigation and mobile layout of the docs site. One commit per issue.

Changes

Fix wide tables zooming out the page on mobile (#262)

• Props tables on the Components and Utilities pages are wrapped in a .table-scroll container, so they scroll horizontally on their own instead of widening the page.
• Long type names in API item headings (e.g. RouteComponentPropsWithData<TParams, TData, TState> on the Types page) can now wrap. This was another overflow source on narrow screens.

Measured at a 390px mobile viewport: before, /api/components laid out at 587px, /api/utilities at 489px and /api/types at 409px. After, every docs page lays out at 390px.

Site-wide documentation sidebar (#261)

• New DocsLayout (sidebar + content). In App.tsx, all doc pages except Home and 404 are wrapped in a pathless route that uses it. Most of the App.tsx diff is re-indentation.
• The sidebar stays visible while scrolling (position: sticky) and lists all pages grouped into Getting Started, Learn, API Reference and Resources. The current page is highlighted and marked with aria-current="page". On first load, the sidebar scrolls so the active link is in view.
• The navigation data lives in a new navigation.ts. The sidebar, the mobile menu and the existing Learn/API in-page navs all read from it.
• At 1024px and below, the sidebar is hidden. At mobile widths, the hamburger menu shows Home plus the full docs navigation and scrolls when it's taller than the screen.
• The in-page Learn/API navs are kept at all widths.

GitHub link (#260)

• Added a GitHub button next to "Get Started" and "API Reference" on the homepage.
• The mobile menu now includes a GitHub link (under Resources). Before, there was no GitHub link at mobile widths.

Verification

• pnpm --filter funstack-router-docs typecheck, pnpm lint and pnpm format:check pass, and the docs site builds (Node 24).
• Checked the built site in headless Chromium:
  • the pre-rendered HTML includes the sidebar;
  • clicking a sidebar link navigates client-side and updates the highlight;
  • the mobile menu closes after a link is tapped;
  • no console errors;
  • no horizontal overflow at 390px on any docs page.

Closes #260
Closes #261
Closes #262

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https://claude.ai/code/session_01HFdmEE6GBwx33cNXgjcv7d

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