Explain process for replacing a model entirely #1218
Description
The current "static" model documentation explains how to run a static model where the first time step is used to update the other models for the whole run, but not how to replace a model entirely with input data.
This is done by removing a model completely from the configuration, and providing all variables the model generates as input data.
(this is really documentation for developers/advanced users, but I think it's worth adding to the docs in the current push as Masters' students have asked me how to do it in the past and I had forgotten how to it when I had to sort it for my own research)