Document when and how to `.git-blame-ignore-revs`

[jieyouxu]

I should add some docs in the dev-guide about when and how to add entries to .git-blame-ignore-revs

Originally posted by @jieyouxu in #130736 (comment)

Some large reformatting or refactoring PRs that don't otherwise contain functional changes might be good candidates for .git-blame-ignore-rev (but want to be conservative about which commits qualify so we don't miss out on commits that would benefit from being present in blame)