Add "updated at" information

Please make transparent **when** the data used in the visualization has been **updated**. A link to the source might be helpful, too. I think it would be suitable to place the date at the bottom of the page as indicated in the following screenshot.


# Implementation thoughts

I already had a look at the code and saw that you exclude the water quality data (_data/all.json_) from version control. I saw you have a shell script to download the data. Is there a cron job running on the server to regularly fetch updated data? If so, would it be possible to update a time stamp as well?

The other option I see is to read the "_Aktualisiert:_" field (shown in the following screenshot) from the [data source](https://daten.berlin.de/datensaetze/liste-der-badestellen) and update the website accordingly if it changes.


Start by reading _data/all.json_ and locate the shell script that downloads the water-quality data; inspect how the visualization page currently gets its data and whether a scheduled fetch exists. Done means the page shows the source's update date and, if supported, a source link at the bottom, with the refresh path documented or verified.

Written by the indexing model from the issue text.