Move documetation for reset_approvals endpoint

What does this MR do and why?

Contributes to #353097 (closed)

Problem

We describe merge request approvals endpoints in scope of "Merge Request Approvals API" documentation. However, "Reset approvals of a merge request" section is defined in "Merge Requests API" docs.

Solution

Move documentation for reset_approvals endpoint to "Merge Request Approvals API" section.

MR acceptance checklist

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