Revise building a release instructions

Re: https://github.com/biopython/biopython.github.io/blob/master/wiki/Building_a_release.md aka https://biopython.org/wiki/Building_a_release

This will probably fall to me, but the website changes have knock on implications for doing a Biopython release - the uploads are now done via https://github.com/biopython/DIST to appear under https://biopython.org/DIST/ - and we should also report their checksums in the email and blog release announcements.