Revise ./user/project/merge_requests/index.md for Code Review

As part of the ongoing cleanup for this stage and group described in #323644 (comment 530869621), the technical writer for this group will review and revise this page to meet current style guidelines and correct any deficiencies identified by the PMs / EMs.

Note: this work is lower in priority than feature work. The goal is to set smaller, achievable goals per milestone to ensure this work does not slip.