docs: stand up MkDocs Material Pages site

[joshua-temple]

Stands up a GitHub Pages documentation site for cascade using MkDocs Material.

• Adds mkdocs.yml with the Material theme, full nav across all seven existing docs files (Getting Started, Manifest Reference, Callback Contract, Workflows, CLI Reference, Architecture, Versioning & Schema), and MkDocs extensions for code highlighting, Mermaid fences, and tabbed content
• Adds docs/requirements.txt pinning mkdocs-material==9.6.14
• Adds .github/workflows/pages.yml: builds on every push to main or PR touching docs/**/mkdocs.yml; deploys to Pages on push to main only; all actions SHA-pinned; least-privilege permissions (pages: write + id-token: write scoped to the deploy job only)
• Fixes three broken ../CHANGELOG.md / ../SECURITY.md relative links in versioning.md that pointed outside the docs tree -- replaced with absolute GitHub URLs; mkdocs build --strict now passes clean

Enabling Pages in repo Settings (source: GitHub Actions) is a separate manual step after merge.