feat(v2): support external links and linking to docs from other sidebars (#1052)

* feat(sidebar): support external links and linking to docs from other sidebars

* Update styles.css

v2/lib/theme/Sidebar/SidebarLink.js

@@ -0,0 +1,24 @@
+import React from 'react';
+import {NavLink} from 'react-router-dom';
+import classnames from 'classnames';
+
+import styles from './styles.css';
+
+export default function SidebarLink({href, label}) {
+  const isExternal = /^(https?:|\/\/)/.test(href);
+  const Link = isExternal
+    ? // eslint-disable-next-line jsx-a11y/anchor-has-content
+      ({to, activeClassName, ...linkProps}) => <a {...linkProps} href={to} />
+    : NavLink;
+
+  return (
+    <li className={styles.sidebarListItem}>
+      <Link
+        activeClassName={styles.sidebarLinkActive}
+        className={classnames(styles.sidebarLink, styles.sidebarItem)}
+        to={href}>
+        {label}
+      </Link>
+    </li>
+  );
+}