doc(guide): add concepts

docs/src/example.js

@@ -58,10 +58,10 @@ exports.Example.prototype.addSource = function(name, content) {
 };
 
 exports.Example.prototype.toHtml = function() {
-  return '<h1>Source</h1>\n' +
+  return '<h2>Source</h2>\n' +
           this.toHtmlEdit() +
           this.toHtmlTabs() +
-          '<h1>Demo</h1>\n' +
+          '<h2>Demo</h2>\n' +
           this.toHtmlEmbed();
 };
 

docs/src/templates/css/docs.css

@@ -145,6 +145,11 @@ ul.events > li > h3 {
   font-family: monospace;
 }
 
+.center {
+  display: block;
+  margin: 2em auto;
+}
+
 .diagram {
   display: block;
   margin: 2em auto;
@@ -175,3 +180,7 @@ ul.events > li > h3 {
   color: white;
   text-decoration: none;
 }
+
+.clear {
+  clear: both;
+}