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: https://github.com/orzih/mkdocs-with-pdf/issues/93#issuecomment-1545928952

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).