Redirect rebase before merge docs to fast forward merge

https://docs.gitlab.com/ee/workflow/rebase_before_merge.html is no longer supported. Instead, we introduced https://docs.gitlab.com/ee/user/project/merge_requests/fast_forward_merge.html which provides the rebase function.

• Edit workflow/rebase_before_merge.md according to http://docs.gitlab.com/ee/development/doc_styleguide.html#changing-document-location
• Remove link from https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html#squashing-and-fast-forward-merge