website: Added section outlining supported OSs

This new section, added to the Building documentation, outlines what
OSs, compilers, and Python versions are supported. This documents what
is "strongly supported" (i.e., environments we test), as well as
environments in which gem5 should be able compilable and runnable. The
section highlights that docker images, provided by us, may be used to
compile/run gem5 if problems arise.

Change-Id: I0d38c51298ba13b9245aabf8fdc2b4097db6b035
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/29048
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
1 file changed