Mermaid diagrams don't work with the LaTeX builder

Some setup is evidently needed, which implies that RTD builds will also need some extra legwork when Mermaid diagrams are present in the docs (otherwise they just render empty):

```
writing... WARNING: command 'mmdc' cannot be run (needed for mermaid output), check the mermaid_cmd setting
WARNING: command 'mmdc' cannot be run (needed for mermaid output), check the mermaid_cmd setting
WARNING: command 'mmdc' cannot be run (needed for mermaid output), check the mermaid_cmd setting
WARNING: command 'mmdc' cannot be run (needed for mermaid output), check the mermaid_cmd setting
WARNING: command 'mmdc' cannot be run (needed for mermaid output), check the mermaid_cmd setting
WARNING: command 'mmdc' cannot be run (needed for mermaid output), check the mermaid_cmd setting
WARNING: command 'mmdc' cannot be run (needed for mermaid output), check the mermaid_cmd setting
WARNING: command 'mmdc' cannot be run (needed for mermaid output), check the mermaid_cmd setting
WARNING: command 'mmdc' cannot be run (needed for mermaid output), check the mermaid_cmd setting
done
sphinx-sitemap: No pages generated for sitemap.xml
build succeeded, 9 warnings.

The LaTeX files are in _build/latex.
Run 'make' in that directory to run these through (pdf)latex
(use `make latexpdf' here to do that automatically).
make: Leaving directory '/project/docs'
● (docs/ext-fix $=) ~/Documents/workshop-akcano$ which mmdc

```

A good outcome would be an optional tweak that would enable building Mermaid diagrams in the docs both locally and on RTD with little hassle.