[Release Docs] Docs Notes for Competency ID

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This issue adds user-facing documentation for the Competency ID.

Documentation should explain what a Competency ID is, when a course author or admin encounters or needs it, how it is assigned or displayed, and where it appears in the platform (authoring UI, APIs, or both). 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 ID content should be added in the repository.
• Confirm what "Competency ID" refers to here: the internal id, the stable uuid, or a client-facing key (per this repo's identifier convention, these are three distinct fields) — the docs should describe the one users actually see and use.