Add mdBook documentation site (flakes-px5o)
Create a docs/ folder using mdBook to maintain documentation about the flakes configuration. Seed initial content from CLAUDE.md, integrate with Nix flake (packages.docs, apps.docs-serve), and add mise tasks (docs-build, docs-serve).
Summary of Changes
- Created with mdBook structure (book.toml + 6 content pages seeded from CLAUDE.md)
- Added with (nix build) and
- Added and mise tasks
- Added to