Set line-height of superscripts to 0

This changes it so that superscript (and in particular footnote tags)
do not bump the line spacing of previous lines.

Author: ehuss

src/theme/css/general.css

@@ -190,6 +190,16 @@ kbd {
     vertical-align: middle;
 }
 
+sup {
+    /* Set the line-height for superscript and footnote references so that there
+       isn't an awkward space appearing above lines that contain the footnote.
+
+       See https://github.com/rust-lang/mdBook/pull/2443#discussion_r1813773583
+       for an explanation.
+    */
+    line-height: 0;
+}
+
 :not(.footnote-definition) + .footnote-definition,
 .footnote-definition + :not(.footnote-definition) {
     margin-block-start: 2em;