Adjust debian docs for stable buster
Bump stable to buster.
Keep even jessie as oldoldstable. Sensible? Well, it's still in the tor debian repos.
Also, as a separate commit, adjust the tor-experimental-0.3.4.x suite to 0.3.5.x which is what the repos has.
Commits are in docs branch of https://gitlab.com/quite/tor-webwml.git (although the web interface says the project is empty, um!)