update and bundle Python installation instructions

https://bornagainproject.org/21/installation/building/windows/ advises to install Python3.9, though we agreed that for BornAgain 21 we would recommend Py3.11.

Such bugs are almost unavoidable because instructions regarding Py are scattered over too many pages. Therefore let's take all Py installation instructions out of the "install" and "build" sections, and have a "install Python" section directly under "Install or build".