[doc] add alternate way to get our gpg key
We currently say (on https://doc.arvados.org/v2.2/install/packages.html)
# apt-get --no-install-recommends install gnupg # /usr/bin/apt-key adv --keyserver pgp.mit.edu --recv 1078ECD7
The pgp.mit.edu keyserver is quite unreliable. We also publish our key at http://apt.arvados.org/pubkey.gpg.
Update the docs: replace pgp.mit.edu with keyserver.ubuntu.com which seems more reliable. Also add the alternate access method via curl. Finally, stop using apt-key to add the key, it is no longer recommended (and even on Ubuntu 18.04 the new way works, dropping a file in /etc/apt/trusted.gpg.d/).
Also update the path for the public key at rpm.arvados.org (leaving the old path accessible via a symlink) and update the doc accordingly.