media: docs: move soc-camera.rst to staging

As the entire soc_camera driver is on staging to be removed
soon, let's place there its documentation too.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/media/v4l-drivers/index.rst

@@ -61,7 +61,6 @@ For more details see the file COPYING in the source distribution of Linux.
 	si470x
 	si4713
 	si476x
-	soc-camera
 	uvcvideo
 	vimc
 	vivid