Add mdBook infra runbook served by Caddy on control

Docs live in docs/ (DNS, ZeroTier mesh, monitoring), built at Nix-build time and
served as static files over the ZeroTier mesh on control:8080. Commit-to-edit:
change the markdown and redeploy to publish.

docs/src/SUMMARY.md

@@ -0,0 +1,6 @@
+# Summary
+
+- [Overview](./overview.md)
+- [ZeroTier mesh](./mesh.md)
+- [DNS](./dns.md)
+- [Monitoring](./monitoring.md)