Add merge conflict docs

doc/user/project/merge_requests/resolve_conflicts.md

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+# Merge conflict resolution
+
+When a merge request has conflicts, GitLab may provide the option to resolve
+those conflicts in the GitLab UI. (See
+[conflicts available for resolution](#conflicts-available-for-resolution) for
+more information on when this is available.) If this is an option, you will see
+a **resolve these conflicts** link in the merge request widget:
+
+![Merge request widget](img/merge_request_widget.png)
+
+Clicking this will show a list of files with conflicts, with conflict sections
+highlighted:
+
+![Conflict section](img/conflict_section.png)
+
+Once all conflicts have been marked as using 'ours' or 'theirs', the conflict
+can be resolved. This will perform a merge of the target branch of the merge
+request into the source branch, resolving the conflicts using the options
+chosen. If the source branch is `feature` and the target branch is `master`,
+this is similar to performing `git checkout feature; git merge master` locally.
+
+## Conflicts available for resolution
+
+GitLab allows resolving conflicts in a file where all of the below are true:
+
+- The file is text, not binary
+- The file does not already contain conflict markers
+- The file, with conflict markers added, is not over 200 KB in size
+- The file exists under the same path in both branches
+
+If any file with conflicts in that merge request does not meet all of these
+criteria, the conflicts for that merge request cannot be resolved in the UI.
+
+Additionally, GitLab does not detect conflicts in renames away from a path. For
+example, this will not create a conflict: on branch `a`, doing `git mv file1
+file2`; on branch `b`, doing `git mv file1 file3`. Instead, both files will be
+present in the branch after the merge request is merged.