doc: replace hlist with column-width class

For long lists of items, it's better to use a multi-column display to
make better use of the screen space.  We used the hlist directive to
accomplish list, but it has a drawback on small (phone) screens because
under the hood, the rendering is done using tables.

Instead, we can take advantage of built-in CSS multi-column support
available in recent browsers.  So, convert uses of the hlist directive
to use an rst-class directive to apply a multi-column class to
the entity. The chosen column-width (18em) gives us a 3-column display
on typical window sizes, but will adjust to more or fewer columns
depending on the actual real estate available.

Also, update the documentation guidelines to mention this change.

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