devguide: move image generation into sphinx setup

devguide: move image generation into sphinx setup

While ReadTheDocs can't execute arbitrary scripts, we can install
mscgen in the container that builds the docs (see .readthedocs.yaml).

Then instead of calling generate-images.sh from the Makefile, move this
into the setup function defined on conf.yaml, which will generate the
scripts as part of a normal Sphinx workflow.

This should give us an image generation solution that works on
ReadTheDocs, and locally within anyones build system provided they have
mscgen installed.