Request maintainer to inform product group after merging

What does this MR do and why?

Our current MR review process allows any Merge Request to be reviewed, approved and merged by any reviewers or maintainers, regardless of the product area.

There are guidelines to request additional domain expert reviews, but they are optional:

https://docs.gitlab.com/ee/development/code_review.html#the-responsibility-of-the-reviewer

Some merge requests may require domain experts to help with the specifics. Reviewers, if they are not a domain expert in the area, can do any of the following:

https://docs.gitlab.com/ee/development/code_review.html#the-responsibility-of-the-maintainer

Because a maintainer’s job only depends on their knowledge of the overall GitLab codebase, and not that of any specific domain, they can review, approve, and merge merge requests from any team and in any product area.

This process is sufficient when the MR is authored by someone who is in the relevant product group.

However, if the MR is authored by someone outside the product group that actively works on that product category, there might be a loophole where the MR could be merged without the relevant product group being aware of it. As a result, there might be side effects that affect other ongoing work in the particular area.

Keeping Iteration in mind, we can keep the existing process to encourage quick iteration and not to block the MR, but request that maintainer inform someone from the relevant product group after merging, so that any follow up can be taken if needed.