feat: List of available datasets to be highlighted

[makram93]

As mentioned here, we would like to have a list of dataset readily available. It should be hghlighted in README and also how to use it when running via pip package. From code perspective, it should also be highlighted in argument parser so that users can understand from running jina-now -h

[joschkabraun]

Make Jina now usable only from command line.