Major refactor of tutorials

[dfm]

This PR will add some new tutorials and move some old ones to the case-studies. I've also switched to the sphinx-book-theme with myst-nb to make it easier to launch or download the executed notebooks.

• Partly addresses: Make imports and warning quashing less opaque case-studies#18
• Partly addresses: Make links to notebooks prominent case-studies#16
• Closes Add a documentation page giving an intro to autodiff #187
• Closes Move astrometry tutorial to case-studies and add a shorter astrometry tutorial #188
• Closes JOSS Review: Posterior Predictive Checks in docs #192
• Closes Add notes about reparameterization to the docs #24