Add script to fetch build website's data-files for local testing

[HannesWell]

and provide documentation about the basic structure of the new build websites and their data file generation.

[HannesWell]

With this, as stated in the documentation, one just has to run

./testDataFetch.sh <buildID>
jwebserver

to replicate any current build locally (e.g. for testing changes to the website).