Add merge methods documentation page
This MR introduces merge methods documentation page that gathers existing information on fast-forward merges and semi-linear history, and shows what the resulting commit history will look like.
Before this MR, there wasn't any page that would have all merge methods explained. Currently, fast-forward is explained in https://docs.gitlab.com/ee/user/project/merge_requests/fast_forward_merge.html, and semi-linear history in https://docs.gitlab.com/ee/user/project/merge_requests/reviews/#semi-linear-history-merge-requests (which looks a bit out of place) and regular merge is not explained. This new page combines existing information and adds missing one. It also has images with example merge commit history.
I've also changed the link next to merge methods in project settings. Currently, it is Learn about commit history which doesn't help in understanding what are the differences between merge methods and what do they actually do. I've changed it to a link to a new merge methods documentation page:
Full screenshot of a new documentation page (click to expand)
