nicer style rules for margin around footnote defs

previous implementation used `:not(.fd) + .fd` and `.fd + :not(.fd)`,
which is not only quite awkward, but the latter rule also applies padding
to an arbitrary element (`:not(.fd)`), which makes it very hard to
override in user styles.

this commit replaces these awkward rules with much nicer
`.fd:{first,last}-of-type`, which also describe exactly our usecase.

src/theme/css/general.css

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