Add branch rules section

What does this MR do and why?

Adds a Branch rules section to Project > Settings > Repository.

Note this MR is part of a bigger effort to move branch protections and MR approval rule configuration under one shared location called "Branch rules".

MR breakdown

title	note
Add branch rules section in repository settings (behind a FF)	👈 this MR
Set up a new Branch rules page (empty)	follow-up
Add Branch rules endpoint to Project routes	follow-up

Screenshots or screen recordings

How to set up and validate locally

1. Enable the branch_rules feature flag.
2. Navigate to Project > Settings > Repository > Branch rules

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 #362205 (closed)