website: Add a Slack invite link to the homepage

Change-Id: I71ee2f5a62a7318eb49c417d12b76428c50dcaed
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/63251
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
1 file changed