Add support for Mermaid diagrams

[EugeneLau0]

graph LR
A-->B

Loading

[rjzupkoii]

Based upon a quick review of the official documentation for Mermaid, this does seem like something that should be possible for the next milestone release.

[rjzupkoii]

Similar request to Issue #1344 for Tikz diagrams, we should pick only one to actually use for the template.

[rjzupkoii]

I did a bit of testing with Mermaid and while I was able to get it up and running locally - it's mostly a matter of adding a script to the template - due to the limitations of GitHub Pages, it doesn't appear that callbacks to the JavaScript used for rendering is allowed. This might get fixed in the future, but for now I'm going to remove this from the v0.9 milestone as a feature.