Add very basic sidebar implementation

Missing stuff that would be supported in readme:
- external links
- children docs
- grouping by tags

I think this is fine for now.

example/preview/src/Preview.jsx

@@ -1,7 +1,9 @@
 const React = require('react');
+const ReactDOM = require('react-dom');
 const PropTypes = require('prop-types');
 const extensions = require('../../../packages/oas-extensions/');
 
+const Sidebar = require('./Sidebar');
 const ApiExplorer = require('../../../packages/api-explorer/src');
 const withSpecFetching = require('../../src/SpecFetcher');
 
@@ -32,6 +34,7 @@ class Preview extends React.Component {
         </div>
 
         { status.length ? <pre style={{ marginLeft: '20px' }}>{status.join('\n')}</pre> : null }
+        { status.length === 0 && oas.info && <Sidebar title={oas.info.title} docs={docs} /> }
         {
           status.length === 0 && (
             <ApiExplorer

example/preview/src/Sidebar.jsx

@@ -0,0 +1,26 @@
+const React = require('react');
+const ReactDOM = require('react-dom');
+
+function Doc({ doc }) {
+  return (
+    <li className="hub-sidebar-page subnav-expanded">
+      <a className="text-overflow" href={`#${doc.slug}`}>
+        <span className={`pg-type type-${doc.api.method}`}>{doc.api.method}</span>
+        {doc.title}
+      </a>
+    </li>
+  );
+}
+
+function Sidebar({ title, docs }) {
+  return ReactDOM.createPortal((
+    <div className="hub-sidebar-category">
+      <h3>{title}</h3>
+      <ul>
+        { docs.map(doc => <Doc key={doc._id} doc={doc} />) }
+      </ul>
+    </div>
+  ), document.getElementById('hub-sidebar-content'));
+}
+
+module.exports = Sidebar;