ci: add docs to release notes

Signed-off-by: Dave Tucker <dave@dtucker.co.uk>

.github/changelog-base.json

@@ -19,6 +19,11 @@
         "title": "## 🤝 Dependencies and 🤖 Generated Code",
         "labels": ["dependencies"],
         "exhaustive": true
+      },
+      {
+        "title": "## 📃 Documentation",
+        "labels": ["documentation"],
+        "exhaustive": true
       }
     ],
     "ignore_labels": [