Add a how-to guide for scenario modelling

This adds a new example SIRV model and shows how to use it to compare
two interventions with different effects.

We use the "sphinx-togglebutton" extension to collapse each code block
in the scenario modelling guide, so that the user can focus on the main
text and results figures, but also expand these blocks to examine any
implementation details of interest.

We also include each top-level page in its own table of contents. For
example, the first section link for the Getting Started tutorial is now
"Getting Started", and "Installation" is now the second link.