Document gitcompile

[briantoby]

Describe use of gitcompile.py in webdocs/sources/compile.rst

[briantoby]

Outline:

• install Python along with GSAS-II's needed packages (in your case you can use the one in /lus/eagle/projects/RECUP/twang/g2full_04292025 via /lus/eagle/projects/RECUP/twang/g2full_04292025/bin/activate), but you will need to use conda install meson cython` to add tools needed for building. I assume gcc & gfortran are already available.

• Download the gitcompile.py file into the location where you want GSAS-II to be installed. The install location will be a subdirectory named GSAS-II under the location where the gitcompile is installed.

• run the script: `python /gitcompile.py'. This should download and compile GSAS-II.