doc/dev/crimson/pipeline.rst: simplify and update to reflect new stages

doc/dev/crimson/pipeline.rst: simplify and update to reflect new stages

This commit updates pipeline.rst to include some basic information about
how the pipeline stages now work.  I've removed the explicit listing of
the different stages as I'd rather readers refer to the actual
implementation for those details to avoid them getting out of date.

I also removed the comparison to classic as the approach has now diverged
quite a bit and I feel that the ordering part is more important to focus
on than the points at which processing might block.

Signed-off-by: Samuel Just <sjust@redhat.com>