fix(v2): Fix MDX docs being considered as partials when siteDir match the _ prefix convention (#5199)

* Add _ to dogfood docs folder to cover against edge case

* Fix edge case with MDX partials when site / content dir contains a _ prefix

* add globUtils tests

* proper dogfooding folder re-organization, all content plugins being used

* refactor dogfooding folder + expose /tests page index

* fix page plugin ignoring options.routeBasePath

website/_dogfooding/docs-tests-sidebars.js

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+module.exports = {
+  sidebar: [
+    {
+      type: 'doc',
+      id: 'index',
+      label: 'Index',
+    },
+    {
+      type: 'category',
+      label: 'Huge sidebar category',
+      items: generateHugeSidebarItems(4),
+    },
+  ],
+};
+
+function generateHugeSidebarItems() {
+  const maxLevel = 4;
+  const linksCount = 5;
+  const categoriesCount = 5;
+
+  function generateRecursive(maxLevel, currentLevel = 0) {
+    if (currentLevel === maxLevel) {
+      return [];
+    }
+
+    const linkItems = [...Array(linksCount).keys()].map((index) => ({
+      type: 'link',
+      href: '/',
+      label: `Link ${index} (level ${currentLevel + 1})`,
+    }));
+
+    const categoryItems = [...Array(categoriesCount).keys()].map((index) => ({
+      type: 'category',
+      label: `Category ${index} (level ${currentLevel + 1})`,
+      items: generateRecursive(maxLevel, currentLevel + 1),
+    }));
+
+    return [...linkItems, ...categoryItems];
+  }
+
+  return generateRecursive(maxLevel);
+}