Add navigation for merge commit message template page

I've created a new doc page for merge commit message template in MR gitlab!64437 (merged). It was released in gitlab 14.5.

It will also hold docs for squash commit message template from MR gitlab!74340 (merged).

I wasn't aware that the navigation has to be added in a separate project. The new page is already live: https://docs.gitlab.com/ee/user/project/merge_requests/commit_templates.html

Here's a screenshot with menu after my change: