docs: flatten sidebar navigation

[SPIKESPIGEL404]

Summary

• render the six docs groups as native Fumadocs section separators
• expose every non-Reference page directly in the sidebar while preserving Reference API-family folders
• add lint coverage that rejects hidden, duplicate, or missing sidebar routes

Validation

• pnpm lint:docs
• pnpm test
• pnpm build
• git diff --check
• browser verification of flat sections and expandable Reference folders