Update documentation to focus on Jupyter Book V2 instead of V1

[choldgraf]

There are many places throughout the documentation that frame MyST as an alternative to Jupyter Book, and focus on the Jupyter Book V1 stack. Once Jupyter Book V2 is released, we should start talking about the two projects in a different way. Jupyter Book 2 (we should probably keep the "2" for a few release cycles) is a distribution of MyST, rather than an "alternative" to it.

[rowanc1]

#1732 is a great first step in this direction. 🚀