share submission documentation

The [model](https://brain-score-language.readthedocs.io/en/latest/modules/model_tutorial.html) and [benchmark](https://brain-score-language.readthedocs.io/en/latest/modules/benchmark_tutorial.html) tutorials from language consist of generic as well as domain-specific parts. I think we should share the generic parts across domains, e.g. by including `core` documentation in the domain-specific repositories ([likely by downloading the `rst` files](https://stackoverflow.com/questions/66495200/is-it-possible-to-include-external-rst-files-in-my-documentation)).