docs: nav structure, dark-mode diagrams, basePath

[pratyush618]

Summary

• Promote top sections (Getting Started · Guides · Architecture · API Reference · More) to persistent sidebar tabs via root: true + tabMode="top". Each tab lands on the section's index.mdx.
• Split shared layout options into baseOptions (docs) and homeOptions (home + primary nav links), removing the duplicate nav strip in the docs sidebar.
• Add a four-card "Documentation sections" grid to the home hero so the primary doc surfaces are reachable in one click from /.
• Fix workflow diagrams that were unreadable in dark mode (low-contrast fills with no color) — replaced ad-hoc style X fill:#hex lines with semantic classDefs pinning fill, text colour, and stroke.
• Convert the node-statuses diagram from stateDiagram-v2 to flowchart TD so dense edge labels stop overlapping.
• Drop duplicate "index" entries from sub-folder meta.jsons — they were rendering as a redundant child link beneath each folder header.
• Make the GH Pages basePath an explicit DOCS_BASE_PATH env var (replacing the NODE_ENV === "production" heuristic), so pnpm build && pnpm start previews work locally without 404 storms.

Test plan

• pnpm --dir docs types:check passes
• pnpm --dir docs lint passes
• pnpm --dir docs build succeeds (full static export)
• pnpm --dir docs start serves at http://localhost:3000/; home, every section landing page, and a representative deep page render with no asset 404s
• On /docs/... pages, the top-tab strip lists all five sections with the active section underlined; sidebar shows only the active section's pages
• Workflow diagrams (caching, conditions, gates, fan-out, building) are readable in both light and dark themes