feat: add Documentation Platform as v2.1.0 and reorganize roadmap

**Major Changes:**

1. **Removed:** docs/release-notes/ folder
2. **Updated .gitignore:** Add /synapse-docs/ (separate git repo for Docusaurus site)
3. **New v2.1.0 - Documentation Platform** (now first milestone):
   - v2.0.1-dev: Docusaurus setup & structure
   - v2.0.2-dev: Migrate & enhance existing docs from /docs
   - v2.0.3-dev: Installation & deployment guides
   - v2.0.4-dev: API documentation & examples (OpenAPI/Swagger)
   - v2.0.5-dev: Developer & contributor documentation
   - v2.0.6-dev: Deployment & hosting (GitHub Pages/Netlify)

4. **Version shifts:** All subsequent milestones shifted by 1:
   - Package Restructure: v2.1.0 → v2.2.0
   - Observability: v2.2.0 → v2.3.0
   - Performance: v2.3.0 → v2.4.0
   - Advanced Agents: v2.4.0 → v2.5.0
   - Security: v2.5.0 → v2.6.0
   - Plugin Ecosystem: v2.6.0 → v2.7.0
   - Multi-Tenancy: v2.7.0 → v2.8.0
   - Infrastructure: v2.8.0 → v2.9.0
   - Frontend: v2.9.0 → v2.10.0
   - Analytics: v2.10.0 → v2.11.0
   - Release Hardening: v2.11.0 → v2.12.0

**Philosophy:** Documentation-first approach ensures existing system is well-documented before major architectural changes.

**Docusaurus Structure:**
- Separate git repo at /synapse-docs (gitignored from main repo)
- Comprehensive guides (Getting Started, Core Concepts, Deployment, API Reference)
- Interactive examples, diagrams, code tabs
- Versioned docs (v2.x vs v3.x)
- Algolia DocSearch integration
- Deployed to GitHub Pages/Netlify

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: FTMahringer

.gitignore

@@ -15,3 +15,6 @@ Thumbs.db
 
 # Environment
 .env.local
+
+# Documentation site (separate git repo)
+/synapse-docs/