docs: add Tier 3 reference and operations pages

- reference/settings: complete settings hub reference (all 7 tabs —
  Account, System Limits, Notifications, Appearance, Developer, Nodes,
  App Store)
- operations/troubleshooting: 1:1 path rule failures, Docker socket
  permissions, login errors, WebSocket proxy config, offline remote
  nodes, password reset, health endpoint, container logs
- operations/backup: what to back up (DATA_DIR SQLite + COMPOSE_DIR),
  hot backup via sqlite3, cron example, restore steps, host migration
  walkthrough
- mint.json: populate Reference and Operations nav groups

docs/mint.json

@@ -37,11 +37,16 @@
     },
     {
       "group": "Reference",
-      "pages": []
+      "pages": [
+        "reference/settings"
+      ]
     },
     {
       "group": "Operations",
-      "pages": []
+      "pages": [
+        "operations/troubleshooting",
+        "operations/backup"
+      ]
     }
   ]
 }