Looks like it works (local builds are usable, tested using ./build_docs.py --quick --build-root build_root --www-root www --log-directory logs --group $(id -g) --skip-cache-invalidation as documented in the README, on my laptop, beware it take hours).

The current implementation works without modifications on cpython, so it'll work on old branch we don't want to pollute with this, and we'll be able to clean the master branch later if this PR gets adopted.

Please let me merge this PR myself, so I can closely watch how it goes on docs.pyhon.org immediatly after the merge (as salt will deploy this automatically after merging), to catch potential issues as early as possible, but reviews are appreciated!