chore: no warning if array.target_shard_size_bytes is set with shards="auto"

[ilan-gold]

I started a zulip discussion here and I feel like the following PR is reasonable. Using sharding simply to make file systems more performant while viewing the subchunks as the real "units" seems to be a nice use-case and if you opt-in to "auto" with a target size, maybe you shouldn't get a warning?

We could in theory also document the strategy of "pack chunks in equally along all dimensions" but is there really any appetite to change that? That stands in contrast to the current "auto" behavior (without a target size) of 2 chunks per dimension in the shard, which does seems like the sort of thing that could change.

TODO:

• Add unit tests and/or doctests in docstrings
• 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)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.98%. Comparing base (c966cfa) to head (1f827ca).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3883   +/-   ##
=======================================
  Coverage   92.98%   92.98%           
=======================================
  Files          87       87           
  Lines       11246    11247    +1     
=======================================
+ Hits        10457    10458    +1     
  Misses        789      789           

Files with missing lines	Coverage Δ
src/zarr/core/chunk_grids.py	96.27% <100.00%> (+0.01%)	⬆️

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