Add link to docs about conflicts in the resolve conflicts modal

From !73565 (comment 722054762)

we display this modal when users click on Merge locally. Does the content in the documentation add any value to what we already display?

Merge widget	Modal

IT DOES. It explains what to do if the git merge --no-ff line in step 3 fails. (If that line fails, the user must resolve the conflict before moving to step 4.) Item 3 in https://docs.gitlab.com/ee/user/project/merge_requests/conflicts.html#resolve-conflicts-from-the-command-line ("Open the conflicting file in your preferred code editor") details what the user must do before step 4 in the modal becomes possible.