docs: add generated GitHub Pages handbook

[PeterRamotowski]

Summary

• add a generated MkDocs Material documentation site backed by the repository's existing canonical YAML and normative Markdown
• add desktop tabs/sidebar navigation and a mobile drawer-oriented information architecture
• add system-aware light/dark palettes with manual toggles
• add filterable catalogue pages for principles, Core Skills, profiles, modifiers, languages, frameworks, and evaluation scenarios
• add a GitHub Pages workflow that validates the repository, builds documentation on pull requests, and deploys only from main
• add local make docs and make docs-serve targets

Design constraints

The website is presentation-only. principles/entries/*.yaml and the existing normative Markdown remain authoritative; .site-docs/ and .site-build/ are generated and ignored.

The 72 principles are intentionally not expanded into the global navigation tree. On mobile that would create an excessively long drawer, so they are exposed through a searchable/filterable catalogue and the site-wide search instead.