Document how to populate a dataset

Refs https://redmine.teklia.com/issues/7097, and useful for crosslang + BNF

The dataset page lacks a section about how to populate a dataset using two methods:

1. through the frontend, using selection or direct add to dataset
2. through arkindex elements ml-split: simply mention the existence of the tool, and redirect towards https://cli.arkindex.org/elements/#creating-data-splits-for-machine-learning