Add support for Mermaid charts to Doxygen

[azrogers]

As per #997, this PR adds support for Mermaid charts in Doxygen. It allows Mermaid charts to be added to the doc/diagrams folder and included in comments using the @mermaid{filename} alias. You can test it out by grabbing some Mermaid code, for example from this bit of their documentation, putting it in doc/diagrams/test.mmd, and adding a @mermaid{test} command to any Doxygen comment. After generating the docs, you should see the chart show up where the command was placed.

I've put this into a draft PR for the moment because somehow these changes have resulted in the sidebar disappearing. I believe this is a result of modifying Doxygen's footer.html, but I can't tell exactly why it would have this effect. Needs to be fiddled with.

Fixes #997

[azrogers]

For whatever reason, looks like reordering the footer.html and Doxygen settings made the treeview show up again. Black magic.

[azrogers]

For reference, the icon pack I chose here is Mingcute, chosen because it's Apache licensed, meaning we can use it without attribution, and it has a good mix of having a lot of icons to choose from, and not being as big as some (the material-symbols icon set is almost 7MB!). If anyone has any preferences for other icon sets to choose, there's no reason we can't swap them out.

[kring]

I made a couple of small changes...

1. CMakeLists.txt was clobbering the previous value of DOXYGEN_EXAMPLE_PATH, which broke code samples added with @snippet. I just made it append the diagrams path instead.
2. The icons didn't work when the docs were loaded from a file:/// URL, because browser cross-origin rules forbid fetching file from the local file system with fetch. I fixed this with an old school trick: renamed the .json to .json.js and loaded it via a script tag instead.

Otherwise, looks great, merging!