Update docs for getting started using Selenium Server

[garg3133]

Starting from the next release of create-nightwatch, we won't be installing @nightwatch/selenium-server through npm init nightwatch command but it is something users would need to do by themselves.

So, we should update the following doc page accordingly: https://nightwatchjs.org/guide/quickstarts/create-and-run-a-test-with-selenium-server.html