Add links to motor shield manuals to docs 

Add a link in the docs to the [ESP-12E Motor Shield manual](https://www.gitbook.com/book/smartarduino/user-mannual-for-esp-12e-motor-shield) and to the [NodeMCU Wi-Fi car manual](https://www.gitbook.com/book/smartarduino/user-manual-for-wifi-car-by-nodemcu-doitcar-) (an application of the motor shield)