add tutorial on how to add new datasets

[dkrako]

Description of the problem

Currently there is not a step by step guideline on how to contribute datasets to the dataset library.

Description of a solution

Although the contributing.md is a good place to start adding this information, I think it would be best to link from there to a tutorial in our readthedocs page.

The CONTRIBUTING.md could include a link and maybe some more broad description, that we provide a metalibrary of downloadable datasets which are not hosted / provided by us but downloadable via pymovements. More general advices could also be included, like that a fork needs to be created if there's no invite for developing privileges yet.

What the CONTRIBUTING.md could also include is how to test the dataset via tox:

tox -e integration -- tests/integration/public_dataset_processing_test.py::test_public_dataset_processing[PoTeC]

Minimum acceptance criteria

• add dataset contribution step by step guide to CONTRIBUTING.md

[SiQube]

I close this in favour of #379 (duplicate). we also have to think about this when merging #771