website: renamed the markdown files under learning gem5

This is necessary, otherwise the next and previous buttons just
responded alphabetically.

Change-Id: I9767d77ad427ef7dcc5587daf007c3f299b82ee0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/31236
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
27 files changed