doc: add font size for h4 in docs (#99)

Author: hakon-molven-4ss

docs/_static/css/custom.css

@@ -92,6 +92,10 @@ h3 {
   font-weight: 700;
 }
 
+h4 {
+  font-size: 18px;
+}
+
 /* Keep link color for visited links */
 a:visited {
   color: var(--pst-color-link);