Add complex list margins, use margin-less items for ul.simple margins

0.5.0 was moving in the direction of treating everything like a simple
list. 0.4.3 had some bugs that made complex lists look closer to the
spacing that complex lists use. This resolves both and adds specific
item margins for both cases.

See https://docutils.sourceforge.io/sandbox/html4strict/data/simple-lists.html

docs/demo/lists_tables.rst

@@ -130,6 +130,28 @@ Field list
 Bullet Lists
 ------------
 
+..
+    Docutils supports two types of lists, "simple" and "complex". Complex lists
+    have item margins, simple lists do not.
+    https://docutils.sourceforge.io/sandbox/html4strict/data/simple-lists.html
+
+Simple
+^^^^^^
+
+- A simple list.
+- There are no margins between list items.
+- Simple lists do not contain multiple paragraphs. That's a complex list.
+- In the case of a nested list
+
+  - There are no margins between elements
+
+    - Still no margins
+
+      - Still no margins
+
+Complex
+^^^^^^^
+
 - A bullet list
 
   + Nested bullet list.

src/sass/_theme_rst.sass

@@ -147,14 +147,23 @@
     list-style: upper-alpha
   .section ol, ol.arabic
     @extend .wy-plain-list-decimal
+  // Complex bullet lists, they have more item margins than simple lists
   .section ol li, .section ul li
     > *
       margin-top: $base-line-height / 2
+      margin-bottom: $base-line-height / 2
       &:first-child
         margin-top: 0rem
     > p
       margin-bottom: 0rem
     > ul, > ol
+      margin-bottom: $base-line-height / 2
+  // Simple lists, no item margins
+  .section ol.simple li, .section ul.simple li
+    > *
+      margin-top: 0rem
+      margin-bottom: 0rem
+    ul, ol
       margin-top: 0rem
       margin-bottom: 0rem
   .line-block