media: doc: pixfmt-yuv: Move all packed YUV formats to common file

The pixfmt-packed-yuv.rst file documents packed YUV 4:4:4 formats, but
is titled generically as "Packed YUV formats". 4:2:2 and 4:1:1 packed
YUV formats are documented in separate files, which can be confusing.

Group all packed YUV formats in pixfmt-packed-yuv.rst, which allows
documenting the 4:2:2 formats in a more concise way.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>