Add Mermaid (and other markdown) support to Docs
We offer Mermaid support in the product, but our docs don't support it, so you end up with pages like https://docs.gitlab.com/ee/user/markdown.html#mermaid where the docs are supposed to show what the rendering looks like, but it doesn't.
If that's somehow impossible, we should at least convert the "Becomes" section to an image showing the rendered Mermaid.