Add Phase 5: explanation documentation (#96)

## Summary
- Create `docs/explanation/` directory with index and three explanation articles
- why-gitops: Philosophy of GitOps for homelabs (memory, rollback, AI context)
- architecture: How pieces fit together (ASCII diagrams of hosts, data flow, secrets)
- security-model: Tailscale zero-trust, 1Password secrets, access control philosophy
- Update docs/index.md with How-to and Explanation section links
- Update exploring-the-docs to link Explanation section

Decision log deferred to future work.

## Deployment and Testing
- [x] Pre-commit hooks pass (including doc-links validator)
- [ ] Build and deploy to docs.ops.eblu.me to verify rendering

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/96

docs/index.md

@@ -9,7 +9,9 @@ Welcome to the BlumeOps documentation.
 ## Sections
 
 - [[tutorials/index | Tutorials]] - Learning-oriented guides for getting started
-- [[reference/index | Reference]] - Technical reference cards for services, infrastructure, and operations
+- [[reference/index | Reference]] - Technical specifications and service details
+- [[how-to/index | How-to]] - Task-oriented instructions for common operations
+- [[explanation/index | Explanation]] - Understanding the "why" behind BlumeOps
 
 ## About