Agree on terminology for "interactive kernel" functionality

[choldgraf]

In #1235 we had a mixup in terminology, and we should align on this as a term.

@choldgraf used "launch button" to mean "a button that takes a reader out of the current page and puts them into a live JupyterHub or BinderHub session".

@stevejpurves used "launch button" to mean "the button that spins up Thebe and powers a page with a Jupyter kernel so it's interactive.

The Jupyter Book docs use "Launch button" as a parent term that applies to both. It doesn't provide a specific word to differentiate "live code with thebe" but implies that it's also a launch button.

We should pick terminology to use for these two different use-cases so that we don't confuse users (or ourselves)

Proposal

• Use Launch to mean directing a user to a live session on JupyterHub/Binder/Colab/etc.
• Use Interact or Power to mean "connecting a MyST page with a Jupyter kernel on the backend to render its content interactive.

Rationale

My primary rationale is that this is roughly the terminology that Jupyter Book has been using for a few years, given that Thebe is less common than direct Binder/JupyterHub launches. I'm not a huge fan of the specific words, if somebody has a better suggestion I'm definitely open to it.