DOC: "collection of Python code" -> "Python package"

...also, drop link to Python website in the summary on the webpage
that is the landing point of http://tulip-control.org
BUILD: Require gr1c >= 0.9.0
DOC: Pointers to other reading and getting help in Intro

This is redundant with content on the main landing page of
http://tulip-control.org (doc/website/index.html in the repository)
However, it seems appropriate for the Introduction page in the
User's Guide.