Document connectors and the upgrade to encrypted connections

- Guides > Connectors: using connections, sharing modes, attribution,
  and admin setup (encryption key and rotation, redirect URIs, Google
  Drive, SharePoint, Confluence, S3, MCP presets).
- Integration pages register CONNECTOR_REDIRECT_BASE_URI as-is; the
  ?provider= suffix never matched what the backend sends.
  VITE_GOOGLE_CLIENT_ID is optional.
- Upgrading: set ENCRYPTION_SECRET_KEY before migrating multi-user installs.

docs/content/Sources/_meta.js

@@ -1,4 +1,8 @@
 export default {
+  "Connectors": {
+    "title": "🔌 Synced Sources (Connectors)",
+    "href": "/Guides/Connectors"
+  },
   "Per-source-configuration": {
     "title": "🎛️ Per-Source Configuration",
     "href": "/Sources/Per-source-configuration"