Add API documentation for member roles endpoints

What does this MR do and why?

• Endpoints added via !96996 (merged)
• These endpoints are currently documented as curl requests in this docs MR: !104322 (merged)
• Documenting them properly as API endpoints makes more sense as customizable_roles is now rolled out to 100% of GitLab.com and will be enabled by default in %15.9 .

Screenshots or screen recordings

How to set up and validate locally

1. Set up GDK to preview docs locally
2. Navigate to http://gdk.test:3005/ee/api/member_roles.html

OR

See review app page: https://main-ee-111087.docs.gitlab-review.app/ee/api/member_roles.html

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.