[doc] Add more details about PyPI
[doc] Add more details about PyPI

Separate downloading from PyPI and GitHub into separate sections. Be
more explicit that the package is on PyPI, and use the PyPI shields.

McCabeJM/pymdptoolbox,sawcordwell/pymdptoolbox,yasserglez/pymdptoolbox,silgon/pymdptoolbox,McCabeJM/pymdptoolbox,sawcordwell/pymdptoolbox,silgon/pymdptoolbox,yasserglez/pymdptoolbox