WIP: tweaks for lmod page
I've started merging the content that was in systems/perlmutter/finding-and-using-software into the lmod page, and tweaking some of the wording there
Preview at https://portal.nersc.gov/cfs/mpccc/sleak/userdocs3/environment/lmod/, not ready for merge yet
@shahzebsiddiqui , @ronnie15 might want to take a look and comment/edit/etc