docs: Add MR review workflow graph to reviews page
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In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/192042#note_2524764039, Evan suggested copying a Mermaid graph over into the user docs:
> Out of scope and non-blocking, but I saw this section: https://docs.gitlab.com/tutorials/merge_requests/homepage/#the-review-process-for-merge-requests, which is excellent, and wondered if it belonged in the main feature page? Just because I think it's a fixed feature, and wouldn't change in different scenarios?
I might want to hold off until the MR homepage https://gitlab.com/gitlab-org/gitlab/-/issues/535244+ is rolled out more broadly.
## Related
- Related to https://gitlab.com/gitlab-org/gitlab/-/merge_requests/192042+ where this was proposed, specifically in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/192042#note_2524764039
- Related to https://gitlab.com/gitlab-org/gitlab/-/issues/535244+
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