Add GraphQL example for branch rules

What does this MR do and why?

Adds a graphql example for listing branch rules for #385758 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. checkout the docs/385758-branch-rules branch
2. visit http://gdk.test:3000/help/api/graphql/branch_rules_for_project.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.