Improve table of contents

- Replace html snippet with _includes/toc.html
- Add FA icon to table of contents header
- Display table of contents on small screens
- Add hierarchy to toc with indents

_includes/_toc.html

@@ -0,0 +1,9 @@
+<section id="table-of-contents" class="toc">
+  <header>
+    <h3><i class="fa fa-book"></i> Overview</h3>
+  </header>
+<div id="drawer" markdown="1">
+*  Auto generated table of contents
+{:toc}
+</div>
+</section><!-- /#table-of-contents -->