Switch to an all-Inkscape pipeline, with PNG export at the pages stage

I used to do most of my drawings with LibreOffice Draw and export to PNG or WebP, but then I switched to using Inkscape and embedding the Inkscape SVG directly in the document.

The SVG pipeline is nicer for editing, as mdbook serve automatically updates without any tedious export process. But it is not so safe for external publishing because...

• Web browsers with bad SVG support are still a thing
• Not everyone has the Roboto font installed, much to my dismay

For the next edition of the school, I would therefore like to...

• Convert all remaining LibreOffice Draw documents to Inkscape SVG using the new file-per-drawing convention, which enables easier mdbook editing and should also be more git-friendly.
• Add a CI job to render all SVGs to PNG (which can be done using inkscape itself, and that's probably safest) then fix up the markdown links from SVG to PNG using something like sed before rendering the final document.