doc: use multi-column display for long lists

A style was recently added that will allow long narrow lists to display
as three columns across the page (with a responsive design that
self-adjusts based on screen width).  This looks much better than a long
list that runs down the page.

Adding this directive before a block (or nested under the directive)
will allow the content to be multi-column:

   .. rst-class:: rst-columns

as explained in
https://docs.zephyrproject.org/latest/guides/documentation/index.html


in the Multi-column lists section.

This PR tweaks a few remaining documents that have such long narrow
lists.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>