MAINTAINERS: update drm/i915 W: and B: entries

The 01.org page has ceased to exist, and the relevant documentation is
now hosted at https://drm.pages.freedesktop.org/intel-docs/

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231110114807.3455739-1-jani.nikula@intel.com