Documentation/gpu: Reference articles on Linux graphics stack

Add two articles on LWN about the Linux graphics stack to DRM's
list of external references. The articles document the graphics
output as a whole, including the kernel side.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240115113908.25897-1-tzimmermann@suse.de