Add richer documentation, supported by Docusaurus

## Motivation

Git-Ape's documentation currently lives in `docs/` as standalone Markdown files (EXAMPLES.md, ONBOARDING.md, DEPLOYMENT_STATE.md, etc.) and a basic README.md. There is no structured documentation site, no searchable wiki, and no versioned docs. As the project grows with manifesto alignment work (#39), the documentation surface area will expand significantly (ADRs, blueprints, org-level patterns, OPA policies, SRE runbooks, etc.).

A Docusaurus site on GitHub Pages provides:
- Searchable, navigable documentation
- Versioned docs tied to releases
- API/skill reference pages auto-generated from SKILL.md files
- Blog section for release notes and updates
- Dark mode, responsive design, and a professional landing page

## Proposed Work

### 1. Docusaurus Setup
- Initialize Docusaurus project in `docs-site/` (or `website/`)
- Configure for GitHub Pages deployment (`Azure.github.io/git-ape-private` or custom domain)
- GitHub Actions workflow for automatic build and deploy on push to `main`

### 2. Documentation Structure

```
docs-site/
├── docs/
│ ├── getting-started/
│ │ ├── introduction.md ← What is Git-Ape
│ │ ├── quickstart.md ← 5-minute first deployment
│ │ ├── installation.md ← CLI plugin + VS Code extension
│ │ └── prerequisites.md ← Required tools and auth
│ │
│ ├── guides/
│ │ ├── deployment-workflow.md ← Full 4-stage pipeline walkthrough
│ │ ├── onboarding.md ← OIDC, RBAC, GitHub environments
│ │ ├── drift-detection.md ← Manual and scheduled drift
│ │ ├── headless-mode.md ← Coding Agent / GitHub Actions
│ │ ├── cost-management.md ← Cost estimation and budget gates
│ │ ├── security-gates.md ← Security analysis and gate enforcement
│ │ └── blueprints.md ← Golden-path reference architectures
│ │
│ ├── reference/
│ │ ├── agents/ ← Auto-generated from .agent.md files
│ │ │ ├── git-ape.md
│ │ │ ├── requirements-gatherer.md
│ │ │ ├── template-generator.md
│ │ │ └── ...
│ │ ├── skills/ ← Auto-generated from SKILL.md files
│ │ │ ├── arm-checkov.md
│ │ │ ├── azure-naming-research.md
│ │ │ └── ...
│ │ ├── workflows/ ← CI/CD workflow reference
│ │ ├── deployment-state.md ← Artifact format and state machine
│ │ └── configuration.md ← All configuration options
│ │
│ ├── architecture/
│ │ ├── overview.md ← System architecture and design
│ │ ├── manifesto.md ← Manifesto alignment and vision
│ │ ├── adrs/ ← Architecture Decision Records (relates to #42)
│ │ └── security-model.md ← Security analysis integrity rules
│ │
│ └── contributing/
│ ├── development.md ← Dev setup, testing, PR process
│ ├── adding-skills.md ← How to create new skills
│ ├── adding-agents.md ← How to create new agents
│ └── adding-blueprints.md ← How to create golden-path blueprints
│
├── blog/
│ └── 2026-04-15-manifesto-alignment.md ← Release notes / updates
│
├── src/
│ └── pages/
│ └── index.tsx ← Landing page with hero, features, CTA
│
├── static/
│ └── img/ ← Logo, diagrams, screenshots
│
├── docusaurus.config.ts
├── sidebars.ts
└── package.json
```

### 3. Auto-Generation Pipeline
- Script to sync agent descriptions from `.github/agents/*.agent.md` → `docs/reference/agents/`
- Script to sync skill descriptions from `.github/skills/*/SKILL.md` → `docs/reference/skills/`
- Run on CI to keep docs in sync with source-of-truth files

### 4. GitHub Pages Deployment
- GitHub Actions workflow: `docs-deploy.yml`
- Trigger: push to `main` with changes in `docs-site/`, `.github/agents/`, or `.github/skills/`
- Build Docusaurus, deploy to `gh-pages` branch
- Optional: custom domain (e.g., `git-ape.dev` or `gitape.azure.dev`)

### 5. Landing Page
- Hero section with Git-Ape logo and tagline
- Feature cards: Security Gates, Cost Estimation, Drift Detection, IaC Scanning, Architecture Review
- Quick start CTA
- Link to manifesto blog post

### 6. Search
- Enable Docusaurus local search plugin (or Algolia DocSearch for larger scale?)
- Index all docs, guides, and reference pages

## Migration Plan
1. Move existing `docs/*.md` content into the Docusaurus structure
2. Keep `docs/` in repo for backward compatibility (symlink or redirect)
3. Update `README.md` to link to the docs site instead of inline docs
4. Update onboarding to reference the docs site

## Files to Create/Modify
- `docs-site/` — New Docusaurus project
- `.github/workflows/docs-deploy.yml` — GitHub Pages deployment workflow
- `.github/scripts/sync-docs.sh` — Agent/skill docs sync script
- `README.md` — Update to link to docs site

## Acceptance Criteria
- [ ] Docusaurus site builds and deploys to GitHub Pages
- [ ] All existing documentation migrated to structured docs
- [ ] Agent and skill reference pages auto-generated from source files
- [ ] Search works across all documentation
- [ ] Landing page with project overview and quick start
- [ ] GitHub Actions workflow auto-deploys on push to main
- [ ] Docs stay in sync with agent/skill source files
- [ ] Blog section with initial post

## References
- Tracking epic: #39
- Related: #42 (ADRs — architecture docs section), #44 (living docs), #46 (org-level patterns docs)
- Docusaurus: https://docusaurus.io/
- GitHub Pages: https://pages.github.com/