added selenium standalone service to the docs [ci skip]
added selenium standalone service to the docs [ci skip]

Pitzcarraldo/webdriverio,monolithed/webdriverio,sebastiendavid/webdriverio,Pitzcarraldo/webdriverio,sebastiendavid/webdriverio,monolithed/webdriverio,testingbot/webdriverjs,testingbot/webdriverjs