Add support for Mermaid diagrams

[gwatts]

Adding diagrams to documentation is a pretty useful thing; Mermaid really helps as it allows you to generate a diagram from simple code, and keep the code as part of the document, allowing the browser to render it on the fly.

This update allows Mermaid documents to be included as fenced code, in the same way as Gitlab supports, so if you happen to be working in that environment, it should be a similar experience.

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[gwatts]

@googlebot I signed it!

[gwatts]

Here's the link to the doc preview with diagram embedded.

[LisaFC]

This is FANTASTIC. Thanks so much for adding it, it's a great feature.

[LisaFC]

Feel free to send a mail to docsy-users@googlegroups.com to let the community know about the feature, I think lots of users would like to try it.

[gwatts]

Thanks for merging @LisaFC ! I'll have a little update once the next version of Mermaid is released to cleanup the javascript some, and also move it into assets/js along with the other theme javascript and cleanup partial/scripts.html