[Release Docs] Docs for competency criteria associations to courses

This issue adds user-facing documentation for associating competency criteria to courses.

Documentation should explain what a competency criteria association is, how a criterion is attached to a piece of content (course final grade or subsection assignment), how the association is configured (rule/threshold), and how it appears to course authors once set. 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 criteria association content should be added in the repository.
• Confirm this doc is written against the finalized UI (depends on Build the UI for Competency Criteria Associations landing first).