Model wise documentation skill v0

Nobody has claimed this yet.

The `ssm-simulators` package needs an overhaul of the documentation to include model-wise explanations.
Currently there is no conceptual level explanation in the documentation concerning what specific model strings map to in terms of model behavior and mathematical model formulation.

This unnecessarily hampers beginner usage of most of the models we already include in `ssm-simulators`.

The deliverable here is:

1. A `skill` that guides the creation of coherent model wise documentation
2. The corresponding documentation for the core set of models that are currently discoverable via HSSM

We will take this as the basis for a comprehensive run toward full coverage on all models included in `ssm-simulators` which we will do via a follow-up issue.

No files, tests, or entry points are named, so first review the existing documentation and the core models discoverable via HSSM. Define the model-wise documentation skill and document each core model's string mapping, behavior, and mathematical formulation; done means the core set is covered and the material can guide the later full-coverage effort.

Written by the indexing model from the issue text.