Effective tutorial landing page ideas

[gully]

Currently the tutorials are grouped into IGRINS and HPF examples. The differences between these two usage patterns are pretty minimal, and probably dissuades newcomers from one group from exploring the tutorials from the others'. Instead, @ericasaw and I agree that we should re-group the tutorials to a more natural landing page view.

The structure of this landing page should be explored, but may look something like this:

Introductory

• HPF Hello World
• IGRINS Hello World

Instrument-specific considerations

• HPF: Sky Subtraction I
• HPF: Sky Subtraction II
• IGRINS: A0V comparison

Beyond plotting ("spectral manipulations" / Interfacing with Specutils?) (TBD!)

• [...]

We may also want to mimic the radis tutorial layout that offers thumbnails for each tutorials.