[Release Docs] Add documentation and release notes for Competency Taxonomy usage type

This issue adds user-facing release documentation for the new Competency Taxonomy usage type.

Documentation should explain what the Competency Taxonomy usage type means, when to use it, how it is selected or assigned, and how it appears in the platform. Documentation should follow the same process used for the previous taxonomy editing work, with docs added to the Open edX documentation repository.

Release notes are not expected to be created as part of this ticket. Axim is experimenting with generating release notes separately, and the team may share those for review if needed.

Documentation Location:

Documentation should be added to the Open edX documentation repository:
https://github.com/openedx/docs.openedx.org

Outstanding/Needs Confirmation:

• Confirm the exact documentation page or section where Competency Taxonomy usage type content should be added in the repository.