Add SILE documentation and fix PocketBeagle 2 branding

SILE should be a reasonable typesetter to try to avoid tools like Canva, Google Slides and other closed source, non-CI, graphical layout tools.

I think we want to avoid PocketBeagle2 to provide more consistent style. Paths with pocketbeagle2 can be ok, but maybe not preferred either. Paths are probably better to be just beagle. Directory within the repository is "PocketBeagle-2" to avoid spaces in filenames.