website: Added "Extending gem5 for ARM" to learning gem5 part 1

The URL of this page was also changed to be consistent with other
"learning gem5" resources.
"/extending_configs"->"/documentation/learning_gem5/part1/extending_configs"

Change-Id: Ia6068e7cbe351b9115ed1300c423baee1f85ea07
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/31294
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