website: Add Kramdown extension for outdated page notices

This commit implements an outdated notice element which can easily flag
outdated documentation.

'{: .outdated-notice}' will generate an outdated notice element wherever
it occurs in a .md file, beginning with

  "Note: This page is outdated."

in bold font. Any text immediately succeeding it will be included in the
notice as a comment.

Change-Id: I987e019ad322fc61a4c5bf4d4d3dd295d3503167
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/26563
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: Jason Lowe-Power <jason@lowepower.com>
3 files changed