website: Add gem5 standard library documentation Change-Id: I572d8052ed0102cd80a9cb089e09b601279c2900 Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/55123 Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu> Maintainer: Bobby Bruce <bbruce@ucdavis.edu> Tested-by: Bobby Bruce <bbruce@ucdavis.edu>