make release-signing key easy to find

[cron2]

I have received a report that "I haven't been able to find the GnuPG signing key on the community website though, nor pointers".

This might or might not be correct, I didn't have time to go searching, but we should ensure it's easy to find, even for non-GnuPG experts (even if the documentation just says "run gpg --recv-keys" we should have such documentation)