docs(contrib): Add link to documentation in dashboard (#12602)

changelog.d/12602.misc

@@ -0,0 +1 @@
+Add link to documentation in Grafana Dashboard.

contrib/grafana/synapse.json

@@ -66,6 +66,18 @@
       ],
       "title": "Dashboards",
       "type": "dashboards"
+    },
+    {
+      "asDropdown": false,
+      "icon": "external link",
+      "includeVars": false,
+      "keepTime": false,
+      "tags": [],
+      "targetBlank": true,
+      "title": "Synapse Documentation",
+      "tooltip": "Open Documentation",
+      "type": "link",
+      "url": "https://matrix-org.github.io/synapse/latest/"
     }
   ],
   "panels": [
@@ -10889,4 +10901,4 @@
   "title": "Synapse",
   "uid": "000000012",
   "version": 100
-}
+}