fix: proper documentation for `auto` in `shards` docs

[ilan-gold]

This PR adds a model for how we might start adding these sorts of types, by applying the paradigm to shards in create_array. We now have a types.md that contains type hints. I'm personally not 100% if exposing a path to core is kosher, but also in theory, people should be putting typing imports behind a TYPE_CHECKING block so that should be ok? I'm just worried about people importing from these paths, and then those paths breaking (see https://github.com/zarr-developers/zarr-python/pull/3355).

I would thus make it super clear that imports of these types are not guaranteed until merging https://github.com/zarr-developers/zarr-python/pull/3355 and then we can fix these two issues in parallel in theory.

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [x] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/user-guide/*.md
• [ ] Changes documented as a new file in changes/
• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)