[nrf fromtree] doc: extensions: move Zephyr domain extension to dedicated package

move domain.py to domain/ to make it easier to keep things organized
and e.g. add dedicated css/js resources, etc.

Signed-off-by: Benjamin Cabé <[email protected]>
(cherry picked from commit 3d7bb30)

Signed-off-by: Sean Madigan <[email protected]>

Author: kartben