Use PyData theme for docs

[jakirkham]

Make use of the PyData theme in the docs.

Previously RTD installed and configured the default html_theme to use sphinx-rtd-theme, which was ok. However RTD recently changed this as explained in this blogpost. As a result, the docs started using the Sphinx default.

Taking this opportunity to more closely align the Numcodecs docs with the Zarr docs, which use the PyData theme ( zarr-developers/zarr-python#1242 ). This should have a more seamless experience for the reader.

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TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)