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/SidebarCategory.js

@@ -0,0 +1,32 @@
+import React from 'react';
+import classnames from 'classnames';
+
+import styles from './styles.css';
+
+export default function SidebarCategory({
+  label,
+  items,
+  subCategory,
+  renderItem,
+}) {
+  const Heading = subCategory ? 'h4' : 'h3';
+
+  return (
+    <div
+      className={classnames(styles.sidebarGroup, {
+        [styles.sidebarSubGroup]: subCategory,
+      })}
+      key={label}>
+      <Heading
+        className={classnames(
+          styles.sidebarItem,
+          styles.sidebarGroupTitle,
+          styles.sidebarGroupCategoryTitle,
+        )}>
+        {label}
+      </Heading>
+
+      <ul className={styles.sidebarList}>{items.map(renderItem)}</ul>
+    </div>
+  );
+}