Documentation: Mention `.git-blame-ignore-revs` somewhere

[DavisVaughan]

Because it's useful and no one seems to know about it
https://bsky.app/profile/simonpcouch.com/post/3luqioffers2t

[DavisVaughan]

Official docs
https://docs.github.com/en/repositories/working-with-files/using-files/viewing-and-understanding-files#ignore-commits-in-the-blame-view

Main reason to document this is to bypass the preconception that "doing one big formatting commit will bork my usage of git blame!". Not if you use this!