Web page revisions: Installation

[briantoby]

There have been some useful comments (in #218 & #260) on how to use the GSAS-II git repo with pip, which have been incorporated into a new documentation page on using pip to install GSAS-II. While at it, the installation sections of the web pages have been revised extensively, with more documentation on use of pixi, compilation and how to setup for GSAS-II code development using a forked GitHub repo.

This was an extensive set of additions and edits, but I'm sure a lot more could be done.