Extend "Start a review" to actions that could signal that the review has started
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=509545)
</details>
<!--IssueSummary end-->
## Proposal
Extend/repurpose the "Start a review" action to all actions that could inform the assignee of a code review that the reviewer has started a review.
This proposal comes from reconsidering the decision that a "review has started" is at the time of the first comment https://gitlab.com/gitlab-org/gitlab/-/issues/457990#note_1984446538. The reason for reconsideration is the fact that if there were to be reminders that the reviewer has to take action on a merge request https://gitlab.com/gitlab-org/gitlab/-/issues/26353 we wouldn't want the system to nudge the reviewer when in fact they had started.
We want to take actions that could imply that a review has started and making it an explicit action on the reviewer side.
There could be multiple actions that indicate a start of review, so there should be consideration to have activity item when a review is started by a reviewer.
Currently "Start a review" is associated with commenting so we will need to reconsider how that behavior works.
## Actions to consider
Curate known actions and explain how we would include it in a "start a review" flow.
- Gathering context of merge request
- Read MR description
- Read related issues
- Reviewing code
- Mark file as viewed
- Checkout code
- Commenting on changes
- Start a review
- Single comment
## Actions out of scope
There might be actions on the merge request page that the user could take but may not mean it is starting a review.
- TBD
issue