DOC: update the getting started

[jankoslavic]

https://github.com/gwtproject/gwt-site/edit/master/src/main/markdown/gettingstarted.md

The getting started tutorial is still relying on devmode which cannot be used with modern browsers (for years).

[lofidewanto]

I would like to see following in the getting started doc (http://www.gwtproject.org/gettingstarted.html):

• Create your first GWT web app: only showing the web browser part, no server part. Later sure we can show the communication client and server but the main point is the web browser part.
• After comparing Vue.js (https://v3.vuejs.org/guide/introduction.html#getting-started), React (https://reactjs.org/docs/getting-started.html) and Angular (https://angular.io/start) I find the one from React or Vue.js is the easiest one. The Getting Started should be very simple.
• I would take the Eclipse plugin, the Ant build tool and the download ZIP out from the Getting Started. Only using Maven plugin (tbroyer) to begin with.
• I would still use the Widgets for the Getting Started example (GWT 2.9.x).
• Setup an IDE would be just using Maven in the chosen IDE. Debugging just using Chrome web browser.
• We should just ignore the webAppCreator (deprecate it, since everything should be based on Maven).
• Remove SDK Learn More (http://www.gwtproject.org/learnmore-sdk.html)
• Remove Configure Eclipse (http://www.gwtproject.org/usingeclipse.html)