website: Update O3CPU backend documentation

This commit add a post about a detailed description about how
O3CPU backend works. The documentation is also updated.

Change-Id: I22485edddda4a3599a372f1b32793be24cee3e66
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/31554
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
2 files changed