Extend docs.joern.io Quickstart example

[johannescoetzee]

The current quickstart example doesn't give a very good idea of what Joern is capable of. Either expanding this example, or adding a few more example queries, to show more functionality (esp. dataflow analysis) would give a much better introduction.

[DavidBakerEffendi]

+1. Crafting more precise queries to reduce false positives and the relationship between something like local/parameter/identifier is also not clear to new users