Implement mermaidcli approach for supporting mermaid diagrams

[domWalters]

For various reasons, I think use of relaxedjs needs to be aborted in favour of a different approach.

See this comment for an example of how to use mermaidcli to render images before the pdf render happens: orzih#93 (comment)

I propose:

• Reverting the relaxedjs patches.
• Adopting the code from the linked comment above.
• Rework it to avoid use of /tmp files.
  • chromium-browser on Ubuntu is shipped as a snap which can't access the /tmp directory and causes this approach to silently fail.
  • The site folder could quite happily be used to store these temp files, or some other build folder (perhaps user specified).

[domWalters]

Apologies that I've been inactive on this repo for a long time.

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FYI, for anyone who reads this, the mermaid-cli branch supports this functionality.

It is currently not merged into develop, but I hope to progress that soon.

I don't want to merge it until I expose enough configuration to make it usable.