Improve (imo) documentation
I started updating the docs to make it more friendly:
- contrib.rst: outdated dependency list, simplified some steps
- prereq: removed the dependency list (again) because it doesn't belong in docs, there is already META.json for that
- update the structure: a lot of advanced features were moved to "advanced" section, the install steps now have a lot less choice for users deb/rpm and apache/nginx and that's all. No mention of PSGI servers, this is advanced stuff
- removed references to apt-get and aptitude: debian now uses apt
OK for you @guimard @clement_oudot ?