Assign doc reviewer directly instead of only pinging

What does this MR do and why?

Currently, TW are pinged to review, but aren't assigned. This can lead to MRs going off the radar of the whole TW team if the pinged person is OOO. Having them assigned, would allow the rest of the team to reassign to someone that's not OOO in that case.

Also, assigning fore review is the common practice.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review