Add related topics section to branches docs

doc/api/branches.md

@@ -231,3 +231,9 @@ Example request:
 ```shell
 curl --request DELETE --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/projects/5/repository/merged_branches"
 ```
+
+## Related topics
+
+- [Branches](../user/project/repository/branches/index.md)
+- [Protected branches](../user/project/protected_branches.md)
+- [Protected branches API](protected_branches.md)

doc/api/protected_branches.md

@@ -529,3 +529,9 @@ Example response:
    "push_access_levels": []
 }
 ```
+
+## Related topics
+
+- [Protected branches](../user/project/protected_branches.md)
+- [Branches](../user/project/repository/branches/index.md)
+- [Branches API](branches.md)

doc/user/project/protected_branches.md

@@ -326,6 +326,12 @@ Protected branches can only be deleted by using GitLab either from the UI or API
 This prevents accidentally deleting a branch through local Git commands or
 third-party Git clients.
 
+## Related topics
+
+- [Protected branches API](../../api/protected_branches.md)
+- [Branches](repository/branches/index.md)
+- [Branches API](../../api/branches.md)
+
 <!-- ## Troubleshooting
 
 Include any troubleshooting steps that you can foresee. If you know beforehand what issues