Streamline the release process

[strk]

At the moment we have an HOWTO_RELEASE document which became far to complex, having a mix of hope in Debbie to do the release for us and being left with having only partial instructions about some artifacts of the release (docs).

This ticket is to refactor it and have a single script (and Makefile target, eventually) to create all the needed artifacts in an unified way.

Once we have such script (could also be make_dist.sh if we see that convenient) then Debbie could just run the script.

I'd like to also take this opportunity to switch from MD5 hashing to SHA256 and make it easy for whoever takes care of releases to cryptographically sign the packages.