Add generated GraphQL documentation

doc/api/graphql/reference/gitlab_schema.graphql

@@ -6005,6 +6005,11 @@ type MergeRequest implements Noteable {
   """
   targetBranch: String!
 
+  """
+  Indicates if the target branch of the merge request exists
+  """
+  targetBranchExists: Boolean!
+
   """
   Target project of the merge request
   """

doc/api/graphql/reference/gitlab_schema.json

@@ -16717,6 +16717,24 @@
               "isDeprecated": false,
               "deprecationReason": null
             },
+            {
+              "name": "targetBranchExists",
+              "description": "Indicates if the target branch of the merge request exists",
+              "args": [
+
+              ],
+              "type": {
+                "kind": "NON_NULL",
+                "name": null,
+                "ofType": {
+                  "kind": "SCALAR",
+                  "name": "Boolean",
+                  "ofType": null
+                }
+              },
+              "isDeprecated": false,
+              "deprecationReason": null
+            },
             {
               "name": "targetProject",
               "description": "Target project of the merge request",

doc/api/graphql/reference/index.md

@@ -886,6 +886,7 @@ Autogenerated return type of MarkAsSpamSnippet
 | `state` | MergeRequestState! | State of the merge request |
 | `subscribed` | Boolean! | Indicates if the currently logged in user is subscribed to this merge request |
 | `targetBranch` | String! | Target branch of the merge request |
+| `targetBranchExists` | Boolean! | Indicates if the target branch of the merge request exists |
 | `targetProject` | Project! | Target project of the merge request |
 | `targetProjectId` | Int! | ID of the merge request target project |
 | `taskCompletionStatus` | TaskCompletionStatus! | Completion status of tasks |