misc: Update documentation and links for GitHub

This changes mentions of googlesource and Gerrit to instead
link to the gem5 GitHub repository, and updates the documentation
to reflect the GitHub review process.

Change-Id: I5dc1d9fcf6b96f9e5116802f938b7e3bb5b09567
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/71878
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
8 files changed