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? 🤔