Improve documentation of stage, group, category, feature labels

What does this MR do?

1. This documents better the current state of the stage, group, category, and feature labels, adding naming and color conventions.
2. The "Subject labels" have been renamed "Feature labels".
3. I've introduced "Facet labels" (e.g. security, database, master:broken, ~"technical debt" etc.) to add facets to an issue/MR.
4. I've also added links to relevant pages in the handbook and triage operations pages.

Relates to gitlab-org/quality/triage-ops#245 (closed) and gitlab-org/quality/triage-ops#247.

This requires https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32056 to have a green pipeline.

Does this MR meet the acceptance criteria?

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content