Extend documentation for ODE model inference

[SamuelBrand1]

In #526 we made the decision to lean toward using jldoctest blocks to test features of ODE inference, but with a view towards extending the proper documentation.

My view there are two main tasks here:

• Making a vignette demonstrating writing and performing inference with a custom model. This could even introduce users to wider options in SciML dynamic systems definition which can interface to ODEProblem object e.g. Catalyst.jl.
• Move documentation from jldoctest blocks to a doc page in the main docs.