Add cross-signing labs flag to develop and document

Part of https://github.com/vector-im/riot-web/issues/11407

docs/labs.md

@@ -67,3 +67,9 @@ An implementation of [MSC2241](https://github.com/matrix-org/matrix-doc/pull/224
 ## New user info panel (`feature_user_info_panel`)
 
 A new implementation of the user & member info panel, designed to share more code between showing community members & room members.
+
+## Cross-signing (`feature_cross_signing`)
+
+Cross-signing ([MSC1756](https://github.com/matrix-org/matrix-doc/pull/1756))
+improves the device verification experience by allowing you to verify a user
+instead of verifying each of their devices.

riot.im/develop/config.json

@@ -20,7 +20,8 @@
         "feature_many_integration_managers": "labs",
         "feature_mjolnir": "labs",
         "feature_user_info_panel": "labs",
-        "feature_dm_verification": "labs"
+        "feature_dm_verification": "labs",
+        "feature_cross_signing": "labs"
     },
     "welcomeUserId": "@riot-bot:matrix.org",
     "piwik": {