Document the convention used for metatensor labels

[Luthaf]

We need to add a documentation page explaining what keys, sample/components/properties and gradient components will be produced by the various rascaline calculators.

I'm not sure if that should be done for each calculator (duplicating some information about e.g. o3_lambda, system, …); or grouped in a single page (making it harder to find this information for a specific calculator).