doc: add course link to site nav

site/theme/template/Layout/Header.jsx

@@ -159,11 +159,23 @@ export default class Header extends React.Component {
             rel="noopener noreferrer"
           >
             <FormattedMessage id="app.header.menu.pro" />
+          </a>
+        </Menu.Item>
+        { isZhCN ? (
+          <Menu.Item key="course">
+            <a
+              href="https://www.yuque.com/ant-design/course"
+              className="header-link"
+              target="_blank"
+              rel="noopener noreferrer"
+            >
+              教程
               <span style={{ display: 'inline-block', position: 'relative', top: -2, width: 6, marginLeft: 8 }}>
                 <Badge dot />
               </span>
             </a>
-        </Menu.Item>
+          </Menu.Item>) : null
+        }
       </Menu>,
     ];