Update locations of MR developer docs in handbook
Updates outbound links to pages in http://docs.gitlab.com/ee/development that were moved as part of Create new "Merge Requests" development docs se... (gitlab-org/gitlab#386620 - closed).
I've marked this work as dependent on the two remaining page moves, just to be absolutely certain we're in the clear before this work merges.
Related issues
Related to gitlab-org/gitlab#386620 (closed)
Edited by Amy Qualls