MAINTAINERS: add link to the Intel Graphics for Linux web site

There's plenty of drm/i915 related hardware and software documentation,
and firmware downloads for the latest platforms.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>