Fix V3 time types

[chuckwondo]

The zarr.core.dtype.npy.time.DateTime64JSON_V3 and TimeDelte64JSON_V3 types erroneously subclassed zarr.core.common.NamedConfig rather than NamedRequiredConfig. This fixes that and also fixes the broken links in their docstrings to point to the correct source documentation for extension types.

• 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 93.26%. Comparing base (6e7f2f3) to head (9575a54).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3955   +/-   ##
=======================================
  Coverage   93.26%   93.26%           
=======================================
  Files          87       87           
  Lines       11721    11721           
=======================================
  Hits        10932    10932           
  Misses        789      789           

Files with missing lines	Coverage Δ
src/zarr/core/dtype/npy/time.py	98.85% <100.00%> (ø)

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