docs: clarify Zarr v3 codec pipeline roles

[mgalore]

Summary

Clarifies how the Zarr v3 codec pipeline maps to filters, serializer, and compressors, resolving the ambiguity that led ArrayBytes codecs such as imagecodecs JPEG-XL to be supplied as compressors. Adds an executable BytesCodec serializer example, updates the relevant API references, and includes a documentation news fragment.

Closes #4159

For reviewers

The change is documentation and docstring-only. The main point to check is that the descriptions accurately distinguish ArrayArray, ArrayBytes, and BytesBytes codec roles and that the explicit serializer example reflects the API.

Author attestation

• I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

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)

Validation

• pytest -q tests/test_docs.py::test_no_unvalidated_blocks tests/test_docs.py::test_test_only_blocks_come_last tests/test_docs.py::test_documentation_examples[user-guide/arrays.md:arrays] (3 passed)
• mkdocs build --strict started successfully but was terminated by the local runner (exit 137) during the complete site build; the targeted executable documentation test above passed.
• markdownlint-cli2 --files docs/user-guide/arrays.md
• towncrier check --compare-with HEAD
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.84%. Comparing base (ab76998) to head (cb161fd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4166   +/-   ##
=======================================
  Coverage   93.84%   93.84%           
=======================================
  Files          91       91           
  Lines       12572    12549   -23     
=======================================
- Hits        11798    11777   -21     
+ Misses        774      772    -2     

Files with missing lines	Coverage Δ
src/zarr/api/asynchronous.py	96.29% <ø> (ø)
src/zarr/api/synchronous.py	92.95% <ø> (ø)
src/zarr/core/array.py	97.85% <ø> (ø)

... and 1 file with indirect coverage changes

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