feat(v2): support rightToc, emoji, slug for docs (#1382)

* add remark-slug and remark-emoji

* implement right TOC remark plugin

* use rehype-slug ecosystem instead of remark for perf

* first rough implementation for right toc

* nits

* remove unwanted changes

* fix left border styling

* remove depths

* inline snapshot

packages/docusaurus-mdx-loader/src/rightToc/search.js

@@ -0,0 +1,46 @@
+/**
+ * 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 toString = require('mdast-util-to-string');
+const visit = require('unist-util-visit');
+const slugs = require('github-slugger')();
+
+// Visit all headings. We `slug` all headings (to account for
+// duplicates), but only take h2 and h3 headings.
+const search = node => {
+  const headings = [];
+  let current = -1;
+  let currentDepth = 0;
+
+  slugs.reset();
+
+  const onHeading = (child, index, parent) => {
+    const value = toString(child);
+    const id =
+      child.data && child.data.hProperties && child.data.hProperties.id;
+    const slug = slugs.slug(id || value);
+
+    if (parent !== node || !value || child.depth > 3 || child.depth < 2) {
+      return;
+    }
+
+    const entry = {value, id: slug, children: []};
+
+    if (!headings.length || currentDepth >= child.depth) {
+      headings.push(entry);
+      current += 1;
+      currentDepth = child.depth;
+    } else {
+      headings[current].children.push(entry);
+    }
+  };
+
+  visit(node, 'heading', onHeading);
+
+  return headings;
+};
+
+module.exports = search;