[Feature]: Update docs for Zarr v3

[rly]

What would you like to see added to HDMF-ZARR?

As part of the migration to the planned Zarr v3, we will need to update some links and documentation:

• Link at the bottom of https://hdmf-zarr.readthedocs.io/en/stable/tutorials/plot_nwb_zarrio.html
• intersphinx_mapping in conf.py
• extlinks in conf.py
• index.rst and surrounding text
• Link at the bottom of overview.rst
• Docs at the top of README.rst

See #257 for references on what to revert.

Is your feature request related to a problem?

No response

What solution would you like?

^

Do you have any interest in helping implement the feature?

Yes.