Add docs-only label to triage output

Problem

The Triage Support bot currently pick up both documentation and code merge requests made by Support team members.

For larger code merge requests, it is quite common to include documentation within the same merge request. If this happens, the Triage Support bot output will show the documentation label but without clicking into the merge request, you would not know it is a code merge request.

This is a problem if you want to scan the Triage Support bot email at a glance and find all code merge requests.

Solution

This MR adds the docs-only label. This label is automatically applied by the GitLab bot user when only documentation changes are detected.

By taking advantage of this in the Triage Support bot, this allows the following new behaviour:

• A pure documentation merge request will include both documentation and docs-only labels
• A mixed code and documentation merge request will include just documentation
• A pure code merge request will have no labels