doc: fix rst-columns display

Using ``.. rst-class:: rst-columns`` wasn't processed correctly because
of an error in the acrn-custom.css file.  Fix that, update the
documentation guidelines, and make use of the multi-column display in
documents where the toctree created a long list.  Now it will
appear in columns.

Also tweaked the toctree listing to use bold for the first-level items
(making a multi-column display look better, particularly when it has
subsections).

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

doc/contribute.rst

@@ -15,6 +15,8 @@ Read about ACRN's high-level design and architecture principles that led
 to the development of the ACRN hypervisor and its components.  You'll
 also find details about specific architecture topics.
 
+.. rst-class:: rst-columns2
+
 .. toctree::
    :maxdepth: 1
 
@@ -36,6 +38,8 @@ project. Here's where you'll find how the development team works and the
 guidelines they (and you) use to contribute code (and documentation) to
 the project.
 
+.. rst-class:: rst-columns2
+
 .. toctree::
    :maxdepth: 1