docs: reorganize navigation and Mermaid fixtures

[Obed0101]

Summary

• expand every Mermaid fixture by default and lock the behavior with a focused regression
• centralize internal implementation paths in docs/source-map.md, including why the runtime directory says opencode
• reorganize docs navigation into use, features/pages, build/maintain, and community sections

Validation

• bun test test/cli/tui/markdown-render.test.ts (94 pass)
• git diff --check
• local relative Markdown-link check

No version bump; this documents the current v0.1.38 behavior.