docs: Add MR review workflow graph to reviews page

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In !192042 (comment 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 [Feature flag] Enable mr_dashboard_list_type_to... (#535244 - closed) is rolled out more broadly.

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