Constrain main nav to main column

Author: tomchadwin

doc/api_custom.css

@@ -177,6 +177,12 @@ div.header {
 	width: 80%;
 }
 
+#main-nav {
+    width: 80%;
+    margin-left: auto;
+    margin-right: auto;
+}
+
 div.qindex, div.navpath {
     width: 80%;
     margin-left: auto;