docs: Add stray design tweaks (#38835)

Tiny little improvements opportunities I noticed today while browsing
the docs.

Release Notes:

- N/A

docs/theme/css/general.css

@@ -127,6 +127,14 @@ h6:target::before {
   width: 30px;
 }
 
+hr {
+  border: 0px solid;
+  color: transparent;
+  width: 100%;
+  height: 1px;
+  background-color: var(--border-light);
+}
+
 /* This is broken on Safari as of version 14, but is fixed
    in Safari Technology Preview 117 which I think will be Safari 14.2.
    https://bugs.webkit.org/show_bug.cgi?id=218076