doc: refactor split cell formatting in table

doc: refactor split cell formatting in table

Refactored split cell in test_pipeline table to allow it to
convert cleanly to PDF.

The Sphinx/Latex converter doesn't handle split cells like the
following:

  +-------------+--------------+
  | Header 1    | Header 2     |
  +=============+==============+
  |             |              |
  |             |              |
  +-------------+              |
  |             |              |
  |             |              |
  +-------------+--------------+

Instead the table was refactored to a simpler format:

  +-------------+--------------+
  | Header 1    | Header 2     |
  +=============+==============+
  |             |              |
  |             |              |
  +-------------+--------------+
  |             |              |
  |             |              |
  +-------------+--------------+

The same information was retained in the table.

Signed-off-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>