docs RDT theme: fix bottom margin of lists items

List items with two ore more blocks are not well rendered. E.g. the gap
between last block (l1-b2) of the first list item and the following list
item (L2) is to small::

    * L1 xxxxxxxxxx
      xxxxxxxxxxxxx

      l1-b2 xxxxxxx
      xxxxxxxxxxxxx
    * L2 xxxxxxxxxx
      xxxxxxxxxxxxx

So that it can be read more liquidly, a distance was added to the last
block (l1-b2)::

    * L1 xxxxxxxxxx
      xxxxxxxxxxxxx

      l1-b2 xxxxxxx
      xxxxxxxxxxxxx

    * L2 xxxxxxxxxx
      xxxxxxxxxxxxx

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>