feat(v2): docs versioning ❄️🔥 (#1983)

* wip: versioning

* wip again

* nits lint

* refactor metadata code so that we can have inobject properties optimization, fix typing

* remove buggy permalink code

* modify versioned docs fixture such that foo/baz only exists in v1.0.0

* refactor metadata.ts so that there is less transformon object

* more refactoring

* reduce test fixtures, refactoring

* refactoring readability

* finish metadata part

* refactor with readdir

* first pass of implementation

* fix mdx laoder

* split generated routes by version for performance & smaller bundle

* test data for demo

* refactor with set

* more tests

* typo

* fix typo

* better temporary ui

* stronger typing & docsVersion command

* add 100% test coverage for docsVersion command

* more test and delete manual docs cut

* cut 2.0.0-alpha.35 docs

* cut alpha.36 instead

* copyright

* delete versioned docs

* stronger test on metadata

* update typo

packages/docusaurus-plugin-content-docs/src/env.ts

@@ -0,0 +1,55 @@
+/**
+ * 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.
+ */
+
+import path from 'path';
+import fs from 'fs-extra';
+import {VersioningEnv, Env} from './types';
+import {
+  VERSIONS_JSON_FILE,
+  VERSIONED_DOCS_DIR,
+  VERSIONED_SIDEBARS_DIR,
+} from './constants';
+
+export function getVersionedDocsDir(siteDir: string) {
+  return path.join(siteDir, VERSIONED_DOCS_DIR);
+}
+
+export function getVersionedSidebarsDir(siteDir: string) {
+  return path.join(siteDir, VERSIONED_SIDEBARS_DIR);
+}
+
+export function getVersionsJSONFile(siteDir: string) {
+  return path.join(siteDir, VERSIONS_JSON_FILE);
+}
+
+export default function(siteDir: string): Env {
+  const versioning: VersioningEnv = {
+    enabled: false,
+    versions: [],
+    latestVersion: null,
+    docsDir: '',
+    sidebarsDir: '',
+  };
+
+  const versionsJSONFile = getVersionsJSONFile(siteDir);
+  if (fs.existsSync(versionsJSONFile)) {
+    const parsedVersions = JSON.parse(
+      fs.readFileSync(versionsJSONFile, 'utf8'),
+    );
+    if (parsedVersions && parsedVersions.length > 0) {
+      versioning.latestVersion = parsedVersions[0];
+      versioning.enabled = true;
+      versioning.versions = parsedVersions;
+      versioning.docsDir = getVersionedDocsDir(siteDir);
+      versioning.sidebarsDir = getVersionedSidebarsDir(siteDir);
+    }
+  }
+
+  return {
+    versioning,
+  };
+}