docs: clarify difference between load and open in docstrings

[SAY-5]

Closes #463. Adds Notes and See Also cross-references to the load and open docstrings explaining that load eagerly reads data into in-memory NumPy arrays while open returns a lazy Array/Group backed by the store, addressing the maintainer's note that "the documentation could definitely be improved here, PR welcome."

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 93.34%. Comparing base (7afdc8d) to head (4a16523).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3984   +/-   ##
=======================================
  Coverage   93.34%   93.34%           
=======================================
  Files          88       88           
  Lines       11819    11819           
=======================================
  Hits        11033    11033           
  Misses        786      786           

Files with missing lines	Coverage Δ
src/zarr/api/asynchronous.py	94.05% <ø> (ø)
src/zarr/api/synchronous.py	92.95% <ø> (ø)

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