Document file blame and file history

Problem to solve

Create documentation for Git file blame and file history.

Further details

I've just noticed that we have no docs for this. We prob took it from granted as a user well-known to Git, but it's worth adding very brief info on this since it's part of the UI.

Proposal

• Document this in https://docs.gitlab.com/ee/user/project/repository/index.html
• Crosslink from https://docs.gitlab.com/ee/api/repository_files.html#get-file-blame-from-repository

Who can address the issue

Anyone. It's ~"Good for 1st time contributors", onboarding TWs, or Community contribution.