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/develop.rst

@@ -7,6 +7,8 @@ Advanced Guides
 Configuration and Tools
 ***********************
 
+.. rst-class:: rst-columns2
+
 .. toctree::
    :glob:
    :maxdepth: 1
@@ -23,6 +25,8 @@ Configuration and Tools
 Service VM Tutorials
 ********************
 
+.. rst-class:: rst-columns2
+
 .. toctree::
    :maxdepth: 1
 
@@ -32,6 +36,8 @@ Service VM Tutorials
 User VM Tutorials
 *****************
 
+.. rst-class:: rst-columns2
+
 .. toctree::
    :maxdepth: 1
 
@@ -48,6 +54,8 @@ User VM Tutorials
 Enable ACRN Features
 ********************
 
+.. rst-class:: rst-columns2
+
 .. toctree::
    :maxdepth: 1
 
@@ -70,6 +78,8 @@ Enable ACRN Features
 Debug
 *****
 
+.. rst-class:: rst-columns2
+
 .. toctree::
    :maxdepth: 1
 
@@ -80,6 +90,8 @@ Debug
 Additional Tutorials
 ********************
 
+.. rst-class:: rst-columns2
+
 .. toctree::
    :maxdepth: 1
 
@@ -91,7 +103,3 @@ Additional Tutorials
    tutorials/sign_clear_linux_image
    tutorials/enable_laag_secure_boot
    tutorials/kbl-nuc-sdc
-
-
-
-