Use excalidraw for our manually-created diagrams

[choldgraf]

Currently we have diagrams embedded as PNGs that are created by various kinds of diagram software. However, PNGs are beefy and not great for git, and they are also a little bit hard to make edits to unless you've got access to the original software used to create the PNG.

In recent years, Excalidraw has become a popular way for maintainers to create diagrams that is a bit more git-friendly, that looks good, and is easy to edit / re-edit by anybody.

In particular, you can export excalidraw to an SVG, this can include embedded excalidraw metadata so that somebody can re-load the SVG into Excalidraw and make edits as they wish. We could then propose changes via edits to an SVG, rather than a new binary PNG file.

Also of note is that excalidraw is open source and generally a pretty well-maintained project.