Add reviewer to merge request to webhook api

What does this MR do and why?

Expose MR webhooks expose reviewers and reviewer_ids field.

Fix outdated doc.
cc @zhanglinjie @daveliu Relates: #300758 (closed)

Doc changes

According this test mr webhook should expose assignee_ids

this test show assgine shouldnt exist

Screenshots or screen recordings

current:

after:

MR acceptance checklist

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