A couple of ideas related to #19191 and #19418

1. Adds a link to numpy-tutorials to the numpy docs nav-bar. This isn't critical, I just wanted to highlight this feature of the pydata sphinx theme in case we wanted to have the tutorials accessible from every page in the numpy docs. Note that I (arbitrarily) chose "Learn" for the link name, but that's probably confusing since there's already numpy.org/learn which points elsewhere. This could be added separately from DOC: Removing tutorials from sphinx documentation #19418 if desired.

2. Illustrates an alternative to the approach taken in DOC: Removing tutorials from sphinx documentation #19418 to address DOC: Removing tutorials from sphinx documentation #19418 (comment) in particular. Instead of having tutorials_index.rst as a stub providing the user with a link to numpy.org/numpy-tutorials, we can instead add external links to numpy-tutorials directly to the toctrees/templates. This is similar to the pattern already used for the installation instructions (see e.g. the toctree in source/user/index). The main advantage of doing it this way is that it requires one less click from the user and integrates numpy-tutorials more tightly with the numpy reference docs.

This PR is just to illustrate ideas via the circleci build artifact, mostly to compare with #19418.