Port of 28458-present-gitlab-version-for-v4-changes-on-docs

EE port of gitlab-org/gitlab-ce!9425

Present GitLab version for each V3 to V4 API change on v3_to_v4.md.

And add Merge Request link to the v3 to v4 documentation. Similar to changelog, mention the Merge Request id & link for each change.