Replace Git Pre-commit Hooks guidance

[timblair]

We're working on a plan to enable secret scanning and push protection on all public GitHub repos within the CO enterprise, should replace the need for pre-commit hooks in a large number of cases, so that guidance will need to be updated once it's clear what the new path is. We'll also need to consider:

• That secret scanning and push protection is only available for public repos.
• Some areas (but not all) have GHAS so can use secret scanning and push protection on non-public repos.