Import sysadmin content from wiki

This is a direct import from the wiki. Many links will be broken, and
plenty of formatting will be dire.
A more useful gitignore
Better front-page copy, and link to sysadmin folder
sysadmin: simplify sysadmin index page
sysadmin: remove introduction, add overview

Remove an overly wordy, prone-to-going-out-of-date introduction, with an
attempt to provide a very broad overview of operated services and
infrastructure.
sysadmin: reorganise "service" pages
Include todos on published site