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.

machines/control/configuration.nix

@@ -4,6 +4,7 @@
     ../../modules/static-ipv6.nix
     ../../modules/monitoring/exporters.nix
     ../../modules/monitoring/server.nix
+    ../../modules/docs.nix
   ];
 
   clan.core.sops.defaultGroups = [ "admins" ];