Add documentation for branch_rules feature

What does this MR do and why?

Adds documentation for branch_rules feature

Screenshots or screen recordings

Page	Doc page

How to set up and validate locally

go to http://127.0.0.1:3000/help/user/project/repository/branch_rules.md

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.

Related to #385758 (closed)