Review mirrors workflow and doc

Some time has passed since mirrors work was handed over to Sysadmins, and now is a good time to review some leftover bits:

• Check why puppet-tails!122 (closed) was not merged and make sure to merge it: puppet-tails!135 (merged)
• Delete dns.lizard:/srv/repositories/mirrors.git
• Review mirrors documentation in sysadmin-private.git
• Review mirrors documentation in the website
• Review workflow for dealing with slow mirrors