docs: Make most headers a bit smaller

Headers are bold already, so the font doesn't need to be that
big to draw attention.

docs/generic.css

@@ -52,22 +52,22 @@ h1, h2, h3, h4, h5, h6 {
 
 h1 {
   margin-top: 0em;
-  font-size: 2em;
+  font-size: 1.6em;
 }
 
 h2 {
   margin-top: 1.0em;
-  font-size: 1.6em;
+  font-size: 1.4em;
 }
 
 h3 {
   margin-top: 1.0em;
-  font-size: 1.4em;
+  font-size: 1.2em;
 }
 
 h4 {
   margin-top: 1.0em;
-  font-size: 1.2em;
+  font-size: 1.1em;
 }
 
 h5 {