refactor(v2): add typing for docs plugin (#1811)

* refactor(v2): add typing for docs plugin

* nits

packages/docusaurus-plugin-content-docs-legacy/src/__tests__/index.test.ts

@@ -0,0 +1,56 @@
+/**
+ * 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 pluginContentDocs from '../index';
+import {LoadContext} from '@docusaurus/types';
+
+describe('loadDocs', () => {
+  test('simple website', async () => {
+    const siteDir = path.join(__dirname, '__fixtures__', 'website');
+    const siteConfig = {
+      title: 'Hello',
+      baseUrl: '/',
+      url: 'https://docusaurus.io',
+    };
+    const context = {
+      siteDir,
+      siteConfig,
+    } as LoadContext;
+    const sidebarPath = path.join(siteDir, 'sidebars.json');
+    const pluginPath = 'docs';
+    const plugin = pluginContentDocs(context, {
+      path: 'docs',
+      sidebarPath,
+    });
+    const {docs: docsMetadata} = await plugin.loadContent();
+
+    expect(docsMetadata.hello).toEqual({
+      category: 'Guides',
+      id: 'hello',
+      permalink: '/docs/hello',
+      previous: 'foo/baz',
+      previous_title: 'baz',
+      sidebar: 'docs',
+      source: path.join('@site', pluginPath, 'hello.md'),
+      title: 'Hello, World !',
+      description: `Hi, Endilie here :)`,
+    });
+
+    expect(docsMetadata['foo/bar']).toEqual({
+      category: 'Test',
+      id: 'foo/bar',
+      next: 'foo/baz',
+      next_title: 'baz',
+      permalink: '/docs/foo/bar',
+      sidebar: 'docs',
+      source: path.join('@site', pluginPath, 'foo', 'bar.md'),
+      title: 'Bar',
+      description: 'This is custom description',
+    });
+  });
+});