Refactored Shifter documentation

I refactored the Shifter documentation (and some related pages, like the python-shifter page).

The main goal of the refactor was to make existing ressources easier to access (ie: the only full instructions on building a container are in the tutorial which is never linked). I also moves things that seemed misplaced (ie: the Dockerfile examples page had instructions on running with VScode that seemed better suited for the FAQ).

It might be worth further thinking about what should be in the main doc page (how to) and what should be in the FAQ.