Jed/docs debian
This addresses some issues I encountered using the packages on recent Debian and tries to be locally consistent.
It also deletes the unmaintained and unreferenced Quick Start Installation Guide.rst, which I had accidentally started editing before realizing it's an outdated (v3.1) near-copy of Installation-Guide.rst.
Thanks @jedbrown. As we are officially supporting Ubuntu 18.04.x/16.04.x and not any other Ubuntu versions, user should be part of Video group only. Render group is recommend for Ubuntu 20.04 and as we are not officially supporting yet, we are not going to take these changes. Once we start supporting Ubuntu 20.04, we will add this in installation guide.
All other minor changes are considered.
If you won't be adding 20.04 soon, perhaps you'd consider adding one section with tips for people using a newer OS? That could mention render without cluttering your description for the older supported releases.