polish up buster site
This polishes up !513 (merged) which moves the whole build process to Debian/buster. This cleans up things so static files that are part of the build process do not get deployed. And it makes the hack scripts that run the builds use multiple CPUs, for large build time improvements. The tasks were already neatly organized per-file, so I just had to make multiple run simultaneously. The also removes some obsolete docs, e.g. fdroidserver!711 (merged)