dep(docs): deprecate experimental v3 support in docs

[jhamman]

Follow up to #1802 adding warnings to the docs about the pending deprecation of experimental v3 code path.

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/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[joshmoore]

One minor change, otherwise 👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.98%. Comparing base (9d046ea) to head (ae499d8).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1807      +/-   ##
==========================================
- Coverage   99.98%   99.98%   -0.01%     
==========================================
  Files          38       38              
  Lines       14659    14656       -3     
==========================================
- Hits        14657    14654       -3     
  Misses          2        2              

Files	Coverage Δ
zarr/convenience.py	100.00% <ø> (ø)

... and 2 files with indirect coverage changes