Episode suggestion: advanced content on editing models

As we build up [options for easier-to-edit {odin} versions](https://github.com/epiverse-trace/epidemics/issues/254) of the scenarios in {epidemics}, it would eventually be useful to have an advanced episode for learners who'd like to tweak the models (as this is a request we get from many who interact with {epidemics}. Note this {odin} option wouldn't change any of the other tutorials, as `model_default` would keep the same inputs and outputs. The only difference would be the model code it points to, and a nicer interface to make tweaks and recompile the mnodel.

In particular, we've narrowed down the conceptual list of potential model edits in this [walkthrough for {epidemics}](https://epiverse-trace.github.io/epidemics/articles/developer-walkthrough.html#adding-or-removing-model-parameters). This could be a useful breakdown for learners, to show many changes are actually just the same thing from a conceptual point of view.

Here's the list of what we thought would cover most things people would need out of a tweaked model:
- Adding or removing model parameters. E.g. adding a uniform, age-independent, background mortality rate that is unrelated to the epidemic itself, or reparameterising $R_0$ and $beta$ inside model equations.
- Modifying compartmental flows without changing the actual compartments. E.g. adding births/deaths, immigration, zoonosis, waning immunity
- Modifying which parameters can be time dependent. E.g. change in infectious period with better isolation or treatment.
- Group-specific infection parameters. E.g. vaccination targeted at certain ages.
- Adding epidemiological compartments. E.g. more complex vaccination or natural history.
- Changing vaccination rates over time. E.g. catch up or pulse vaccination - this was a special category because need to make sure numbers add up sensibly, and vaccines available can be tracked (i.e. when they're used up, can't vaccinate more).