feat(v2): docs plugin initial work (#1327)

* feat(v2): pluginify docs

* feat(v2): implement docs plugin

* fix(v2): fix bugs in docs plugin for translation and versioning

packages/docusaurus-plugin-content-docs/src/order.js

@@ -0,0 +1,72 @@
+/**
+ * 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.
+ */
+
+// Build the docs meta such as next, previous, category and sidebar.
+module.exports = function createOrder(allSidebars = {}) {
+  const order = {};
+
+  Object.keys(allSidebars).forEach(sidebarId => {
+    const sidebar = allSidebars[sidebarId];
+
+    const ids = [];
+    const categoryOrder = [];
+    const subCategoryOrder = [];
+    const indexItems = ({items, categoryLabel, subCategoryLabel}) => {
+      items.forEach(item => {
+        switch (item.type) {
+          case 'category':
+            indexItems({
+              items: item.items,
+              categoryLabel: categoryLabel || item.label,
+              subCategoryLabel: categoryLabel && item.label,
+            });
+            break;
+          case 'ref':
+          case 'link':
+            // Refs and links should not be shown in navigation.
+            break;
+          case 'doc':
+            ids.push(item.id);
+            categoryOrder.push(categoryLabel);
+            subCategoryOrder.push(subCategoryLabel);
+            break;
+          default:
+            throw new Error(
+              `Unknown item type: ${item.type}. Item: ${JSON.stringify(item)}`,
+            );
+        }
+      });
+    };
+
+    indexItems({items: sidebar});
+
+    // eslint-disable-next-line
+    for (let i = 0; i < ids.length; i++) {
+      const id = ids[i];
+      let previous;
+      let next;
+
+      if (i > 0) {
+        previous = ids[i - 1];
+      }
+
+      if (i < ids.length - 1) {
+        next = ids[i + 1];
+      }
+
+      order[id] = {
+        previous,
+        next,
+        sidebar: sidebarId,
+        category: categoryOrder[i],
+        subCategory: subCategoryOrder[i],
+      };
+    }
+  });
+
+  return order;
+};