[linting] Require docstrings for all (public) methods

[szokeasaurusrex]

All public API functions should be documented. To enforce this requirement, we should add a linter check that requires any newly-added functions to contain a docstring.

We should discuss whether it makes more sense to enforce docstrings for all functions or only public API functions. Having docstrings for private functions could help us remember what our internal functions are used for, but maybe we don't need to enforce this as a strict requirement.

[antonpirker]

I think we do not need to enforce it, but just add initial doc strings to all important functions, and then in the future engineers see that we have a lot of doc strings, and will add them too.