Docs: Update index.rst for installation and test changes

[psobolewskiPhD]

Closes #425

This PR updates the instructions for installation and testing at:
https://numcodecs.readthedocs.io/en/stable/index.html

Specifically, it instructs to use pip and not python setup.py install and to use pytest and not nose.
I also changed the way the DISABLE flags are mentioned. I made the changes based on my attempts using arm64 macOS (so no AVX2, no SSE2). With pip install, setting the env flags wasn't needed, so I mention them as an extra note.

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)

[joshmoore]

Workflows launched, cc: @jakirkham

[joshmoore]

This now matches the other merged PRs. Merging but if there are any issues, we can update the mainline and the docs together. Once again, thanks, @psobolewskiPhD.