Add docs for blob blame feature
What does this MR do and why?
Adds documentation for the blob blame feature.
MR acceptance checklist
Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.
How to set up and validate locally
Navigate to http://127.0.0.1:3000/help/user/project/repository/git_blame.html
Related to #430950 (closed)