Add label to skip release note category validation

What does this MR change and why?

The docs-lint release-note-categories CI/CD job validates the categories values in changed release note files against categories.yml. When you edit an older release note, this job can fail because a category has since been renamed or removed from categories.yml.

This MR adds a pipeline:skip-release-note-categories label. When the label is present on a merge request, the job runs when: never, so you can skip the validation without any code changes.

Changes:

• .gitlab/ci/rules.gitlab-ci.yml: add the .if-merge-request-labels-skip-release-note-categories condition anchor and a when: never rule in .docs:rules:release-note-categories.
• doc/development/documentation/release_notes.md: document the label.

This replaces the more complex "pin category validation to a historical ref" approach that used a CI/CD variable and changes to the validation script.

Related links

• Related to Remove categories HTML comments from 19.1 relea... (!244256 - merged)