drm/doc: Add sections about tiny drivers and external refs to intro page

Learning about the DRM subsystem could be quite overwhelming for newcomers
but there are lots of useful talks, slides and articles available that can
help to understand the needed concepts and ease the learning curve.

There are also simple DRM drivers that can be used as example about how a
DRM driver should look like.

Add sections to the introduction page, that contains references to these.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220420072411.15104-1-javierm@redhat.com