More explicit example with numbers for vaccination

[dinacmistry]

A more concrete example showing an example NGM itself, the way you get R0 and infection distribution (which means we need a bit about the eigenvector of the NGM) and the way you think about adding in vaccination by factoring rows by the proportion susceptible remaining.

The example currently in the primer is a bit confusing, since it references K&R but only in that it arrives at the NGM of beta/gamma * N_i / N. The helpful thing from K&R was validating that we understood what exactly was the NGM, and corresponding R0 and infection distribution. Without numbers, the example now is still somewhat unclear.

Originally posted by @paigemiller in #53 (review)

[dinacmistry]

Move material from the approach.md into primer.md since it goes into this type of example.