minor#1206 Refactored the installation docs (fabpot)

This PR was merged into the master branch.

Discussion
----------

Refactored the installation docs

The idea is to get back a simple and straight to the point into chapter. As Composer is now the defacto standard to install PHP deps, I've moved everything related to the other installation methods in a new chapter.

Commits
-------

dd1ead2 tweaked intro chapter
a0efdc9 simplified the intro doc page