docs: MR authors can't merge their MRs

What does this MR do and why?

This MR explicitly states that MR authors can't merge their MRs. I think that this was an implicit rule since October 2022. The current guidelines don't mention that fact https://docs.gitlab.com/ee/development/code_review.html#merging-a-merge-request

I got this information from Update CODEOWNERS to support segregation of duties (!96696 - merged):

Any committer cannot merge code by themselves.

---

This being said, I think having this specific rule is unnecessary. The following rules that are already in place should completely prevent the author from merging changes that haven't been reviewed by a maintainer:

• Author can't approve.
• MR has to have approval from a maintainer
• Approval gets removed if there are any added commits