Add auto-alg and auto-stepper to the docs

[Vilin97]

#351 add autoselection of 1. the stepper (SSAStepper) and 2. the SSA (Direct for small reaction networks) and other SSAs for bigger ones.

The docs and README need to be updated. We should tell the users to use the default stepper and SSA in the introductory docs, but explain how to select solvers and stepper in more advanced places (essentially, those docs do not need to change).