After the DocBook conversion a number of pixfmt description files just
had a number in the filename (pix-fmt-004, 006, etc) which was not very
descriptive.

Rename them.

Note that pixfmt-008.rst was folded into colorspaces-details.rst, so
that file is deleted. It's easier to maintain that way.

Also moved the colorspace sections to the end of the chapter. The old
order was weird: the "Standard Image Formats" section (an intro into
pixel formats) was followed by the colorspace sections instead of the
pixel format descriptions.

Moving it to the end resolved that issue.
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>