Add branch rules page

What does this MR do and why?

Adds a Branch rules page as part of #362205 (closed)

⚠ Note: in this MR we're simply adding an empty 'skeleton' for the branch rules feature, the rest will be added in a follow-up.

MR breakdown

title	note
Add branch rules section in repository settings (behind a FF)	!88279 (merged)
Set up a new Branch rules page (empty)	👈 this MR

Screenshots or screen recordings

How to set up and validate locally

1. Enable to 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)