nicer style rules for margin around footnote defs

previous implementation used `:not(.fd) + .fd` and `.fd + :not(.fd)`.
the latter selector caused many problems:
- it doesn't select footnote defs which are last children
  (this can be easily triggered in a blockquote)
- it changes the margin of the next sibling, rather than the footnote def
  itself, which can also *shrink* margin for elements with big margins
  (this happens to headings)
- because it applies to the next sibling it is also quite hard to
  override in user styles, since it may apply to any element
  
this commit replaces the latter selector with `:not(:has(+ .fd))`,
which fixes all of the mentioned problems.

src/theme/css/general.css

@@ -200,10 +200,12 @@ sup {
     line-height: 0;
 }
 
-:not(.footnote-definition) + .footnote-definition,
-.footnote-definition + :not(.footnote-definition) {
+:not(.footnote-definition) + .footnote-definition {
     margin-block-start: 2em;
 }
+.footnote-definition:not(:has(+ .footnote-definition)) {
+    margin-block-end: 2em;
+}
 .footnote-definition {
     font-size: 0.9em;
     margin: 0.5em 0;