docs: Use bold text for all headers

All headers except for <h1> were already bold: make it bold as
well to increase visual consistency.

docs/libvirt.css

-h1 {
-    font-weight: normal;
-    color: #3c857c;
-}
-
-h2, h3, h4, h5, h6 {
+h1, h2, h3, h4, h5, h6 {
     color: #3c857c;
 }