doc: add documentation for requirement entity (CourseFlow)

[Duy-Nguyen1104]

Description

This PR add a documentation section for the Requirement entity (which is currently lacking), detailing its purpose, structure, and API endpoints in the existing backend documentation file
(The files for requirement can be found at: thoth-tech/doubtfire-api#68)

Type of change

• Documentation (update or new)

How Has This Been Tested?

This change involves documentation only and does not modify any code functionality. Testing was performed by:

• Verifying the markdown syntax renders correctly
• Ensuring table formatting is consistent with existing sections in the documentation

Testing Checklist

• Tested in latest Chrome
• Tested in latest Edge

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings

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