Make section heading language use consistent

# 🐛 Summary #

When working on #57 I noticed that the section headings in `project-setup/branch-protection.md` are inconsistent. Some use a gerund phrase (e.g. "Selecting a skeleton") while others use the imperative mood (e.g. "Create an initial pull request"). I think the latter is preferred.

This is an easy enough change to make, but we will need to ensure that it does not result in any broken links. I believe this document is cross-referenced elsewhere.