Converge tutorials (eliminate duplication)

We have multiple copies of tutorial/starter content. Some of that content is in the the play-xxx-start-example some of the content is in https://www.playframework.com/documentation/2.8.x/HelloWorldTutorial (backed by https://github.com/playframework/playframework/blob/master/documentation/manual/tutorial/HelloWorldTutorial.md).

Each writeup includes snippets maintained on each of the repos. The snippets not only are duplicate across repos, but the twirl templates are also duplicate between the play-xxx-start-example samples.

The welcome.scala.html templates also include introductions/tutorial contents and even copy/pasted snippets from the HomeController and the main.scala.html.

Summing up, as an example, a change in main.scala.html requires sync'ing code in 5 places:

• 2x twirl templates on each of the samples
• 2x welcome pages on each of the samples
• 1x on the snippets of the documentation backing playframework.com's tutorial

Begin by comparing documentation/manual/tutorial/HelloWorldTutorial.md with the play-xxx-start-example content, including welcome.scala.html, main.scala.html, and HomeController snippets. Map the duplicated tutorial and Twirl content across the samples and documentation; done means the tutorial has a single maintained source rather than requiring updates in five places.