Update docs and links to Zarr v2

[rly]

Motivation

Zarr v3 was released and so https://zarr.readthedocs.io/en/stable/ now points to v3 instead of v2. That breaks existing links. This PR updates those links. It also updates the README and related documentation to say that hdmf-zarr supports Zarr v2 and not yet v3.

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.60%. Comparing base (9614553) to head (a1b74c8).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #257   +/-   ##
=======================================
  Coverage   86.60%   86.60%           
=======================================
  Files           5        5           
  Lines        1209     1209           
  Branches      230      230           
=======================================
  Hits         1047     1047           
  Misses        103      103           
  Partials       59       59           

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