DOC: Increase size of headings.

Some time ago, we switch to Carlito, but this font seems to be a bit
smaller than our body font. The default styling makes fourth-level
headings (`h4`) the same size as text, but bold. With the smaller font,
this incorrectly makes the fourth-level headings look smaller than the
text.

To match the body font, Carlito needs a base size of 16px (vs 14px), so
re-scale all the headings using the CSS default em-size * 16px.

Author: QuLogic

doc/_static/mpl.css

@@ -315,20 +315,32 @@ p {
 h1 {
     margin: 0.5em 0em;
     padding-top: 0.5em;
-    font-size: 2em;
+    font-size: 32px;
     color: #11557C;
 }
 
 h2 {
     margin: 0.5em 0 0.2em 0;
     padding-top: 0.5em;
-    font-size: 1.7em;
+    font-size: 24px;
 }
 
 h3 {
     margin: 0.2em 0 0.1em 0;
     padding-top: 0.5em;
-    font-size: 1.2em;
+    font-size: 18.72px;
+}
+
+h4 {
+    font-size: 16px;
+}
+
+h5 {
+    font-size: 13.28px;
+}
+
+h6 {
+    font-size: 12px;
 }
 
 h1, h2, h3, h4, h5, h6{