chore: restructure pre-commit and clarify instructions on fixing existing changes

Following on from some discussions in Holberton team, where there's been some confusion about the initial steps to get pre-commit set-up on an existing project I've had an attempt at clarifying the instructions. This includes:

• restructuring to make more explicit that you should have a set of standard hooks, plus some language-specific checks
• more detail on why running pre-commit run --all-files is helpful, particularly in the context of CI
• adding a .git-blame-ignore-revs file