Add separate on-page navigation sidebar (#475)

lib/core/nav/OnPageNav.js

@@ -0,0 +1,40 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+const React = require('react');
+
+const siteConfig = require(process.cwd() + '/siteConfig.js');
+const getTOC = require('../getTOC');
+
+const Link = ({hashLink, text}) => <a href={`#${hashLink}`}>{text}</a>;
+
+const Headings = ({headings}) => {
+  if (!headings.length) return null;
+  return (
+    <ul className="toc-headings">
+      {headings.map((heading, i) => (
+        <li key={i}>
+          <Link hashLink={heading.hashLink} text={heading.text} />
+          <Headings headings={heading.children} />
+        </li>
+      ))}
+    </ul>
+  );
+};
+
+class OnPageNav extends React.Component {
+  render() {
+    const customTags = siteConfig.onPageNavHeadings;
+    const headings = customTags
+      ? getTOC(this.props.rawContent, customTags.topLevel, customTags.sub)
+      : getTOC(this.props.rawContent);
+
+    return <Headings headings={headings} />;
+  }
+}
+
+module.exports = OnPageNav;