docs: rename demos and make them more use case oriented

Currently the demos are named based on the tech that is used in them. This is good if you're a potential user that already has a Stack of tools that you want to migrate, or if you already have a deeper technical understanding of the tools you might need.

I would propose that we instead rename i.e. nifi-kafka-druid-water-level-data to "Real time stream data analysis and visualization" or something like that. Less focus on the tools and this particular dataset (water levels) and more on the general problem it solves. This way it is more interesting to people that might not know this specific stack, but also to people that might have an existing pipeline that does just this, but maybe with different tools. They might still be interested in using stackable and also swapping out some of their tools in their pipeline while they are at it.

The issue names no files or tests, so first locate the demos and the entry for nifi-kafka-druid-water-level-data. Review the current demo names against the proposed use-case-oriented naming, then rename the relevant demos consistently. Done means the names emphasize the general problem rather than the tools or dataset.