[DAPHNE-#531] Simplified GettingStarted.md.

[pdamme]

Commit message:

• The guidelines on getting started (for users and developers) have grown over time and were not simple anymore.
• Initially, we had a focus on native builds of DAPHNE; hence, the document starts by listing the dependencies etc.
• However, meanwhile, the Docker containers have become the de-facto and simplest way to get started.
• This commit simplifies getting started by prepending quickstart instructions for both users and developers to the beginning of the document, such that these are immediately found.
• These are slightly simplified instructions that should work in most cases.
• For all more complex cases, the original contents of the file is still present (with minor fixes), but under the headline of "Additional Details for Custom Setups".
• The important information from Quickstart.md was merged into GettingStarted.md; thus, Quickstart.md could be removed (for a less fragmented documentation).
• Furthermore, the example hello-world.daph was simplified to really only print "Hello World!"; everything else in this script was rather useless from today's point of view.
• Closes #531.

I invite everyone to share their comments on these changes. If there are no comments, I will merge it on Fri May 17.

[pdamme]

I will merge this PR now. If any comments arise later, we can always change the documentation in follow-up commits.