Installation guidelines improvements

[mdesco]

Helping a non-expert install scilpy. I find our document in "Other tools" of the scil documentation better then our scilpy guidelines.

Before we tell the user to run pip install -e .
Can we please add the git clone line + cd scilpy before. Otherwise, a non-expert does not even know that one needs to clone before doing anything...

Also, I'm curious if we should not recommend a
pip install scilpy for non-experts on the scilpy readthedocs?