Default chunks fix

[d-v-b]

a few changes in here
docs:

• improve some prose in the docs
• fix a broken link in docs

ArraySpec.from_array

• When chunks is not "auto", use guess_chunks from zarr-python instead of defaulting to a single chunk. A single chunk is completely wrong for large arrays.
• expand the function signature of from_array to explicitly list the parameters, and make each one default to "auto", which dispatches to functions that attempt to infer an attribute value from the input data. So an array with an attrs attribute will have the attrs propagate to the resulting ArraySpec.

to_zarr

• make overwrite an explicit keyword-only argument

support for python 3.9

• import TypeAlias from typing_extensions
• add some little pydantic dependency that enables the modern union | operator