Serial commas

[bjnath]

There's precedent in the NumPy site for use of the serial comma -- that is, a comma before the final 'and' in a list of three things or more:

dev/index.rst:240:
the documentation. Matplotlib, SciPy, and IPython are also required.

dev/conduct/code_of_conduct.rst:37:
2. Be empathetic, welcoming, friendly, and patient. We work together to resolve


The new site does not use it:

./public/index.html:670:
Several of these newer libraries such as CuPy, Sparse and Dask,

./public/index.html:205:
NumPy offers comprehensive mathematical functions, random number generators, linear algebra routines, Fourier transforms and more.

Based on precedent (and the Google guide, if we adopt it), the new site should use serial commas too.

I'd be willing to write the PR.

[bjnath]

I added serial commas in the Ecosystem edit of PR #242 but haven't checked them sitewide.

[rgommers]

I've probably been guilty of using this inconsistently. +1 for using it everywhere, and following the Google style guide.