Disable changelog check on docs-only MRs

What does this MR do?

This MR extends the changelog Danger rule so that it doesn't request a changelog entry when changes are docs only.

Our documentation states that:

Any docs-only changes should not have a changelog entry.

So I assume that we can safely disable this check when the MR has docs-only

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Closes #37340 (closed).