Docs: do not use backstage label for silencing the Danger bot
Follow up from !20583 (comment 249292250).
The problem
We have been using ~backstage to silence the Danger bot warning to add a changelog.
But GL documentation is considered code, is part of the feature, and is part of the DoD. It's not really ~backstage.
The solution
Make docs-only to silence the Danger bot instead of using ~backstage.
@gl-docsteam do you have reasons to use ~backstage for docs-only MRs?