Add user guide

[cliffckerr]

In addition to the API and tutorials, we should have a user guide that explains things like:

• Starsim architecture (interactions between networks, diseases, etc)
• Implementation details (e.g. performance optimizations like Numba and arrays)
• What's needed to build a model (states, parameters, etc)
• More detail on the individual "spokes"

Challenge is to figure out how to sensibly split this content between the user guide and tutorials.

Examples:

• https://scikit-learn.org/stable/user_guide.html
• https://epiverse-trace.github.io/epidemics/ ("Articles")

[cliffckerr]

Related to #815