[docs] Add MkDocs documentation site and deploy to GitHub Pages

[cendhu]

Summary

Set up MkDocs with the Material theme to publish project documentation to GitHub Pages. The project already has comprehensive markdown documentation in docs/ but
no documentation site.

What

• Add mkdocs.yml with Material theme configuration (search, syntax highlighting, navigation tabs, light/dark mode)
• Create docs/index.md landing page with project overview
• Add GitHub Actions workflow to build and deploy to GitHub Pages on push to main
• Organize existing docs under structured navigation:
  • Getting Started
  • Architecture
  • Deployment (Deployment Guide, Performance Tuning)
  • Services (Sidecar, Coordinator, Verifier, Validator-Committer, Query Service, Notification Service)
  • Configuration (TLS, Logging, Metrics Reference, Namespace Policy)
  • Development (Core Concurrency Pattern, Load Generator Artifacts)

Why

Documentation is currently only readable by browsing the repo. A hosted docs site improves discoverability and usability for contributors and operators.

Acceptance Criteria

• mkdocs serve builds locally without errors
• GitHub Actions workflow deploys to gh-pages branch on push to main
• All 16 existing docs are accessible via navigation
• Site is available at the GitHub Pages URL