docs: exectutable update

[henryiii]

Start on #157? Also see #259.

[danielballan]

Let's leave #157 open because I would like to specifically illustrate:

• adding a (matplotlib) figure inline
• executing code in line, with Jupytext

We showed these in the NSLS-II guide, and they seem foundational enough to me to be worth showing directly here in this introduction to Python package documentation.

I might consider moving "Notebooks in docs" to a separate topical guide page. That is a popular thing to do, but I think one step more specialized than the essentials covered on this page. It assumes additional knowledge that scientists approaching this might not have ("What's the meaning of the 'kernel name'? How to I manage executed vs unexecuted notebooks in version control?"). It could be linked from the docs.md page but maybe not included as an essential topic in the first page we show about docs.

But as is this is all a step in the right direction, so I would support merging as is and considering future changes separately. Might need some discussion depending on whether my suggestions resonate.