[documentation] add doc on how to add a new model

[davidschlangen]

It looks like we don't have the information in one place on what it takes to add a new model. Which would need to explan the following:

• if model is not supported by an existing backend (which is unlikely, but not impossible): see howto_add_backend.md (but see #54 ).
• Add entry for model in registry. (This could be the place where this is explained in detail and with examples; the document docs/model_backend_registry_readme.md is more about the schema in general.)
• Now the model is available, for example to run the benchmark with it as explained in docs/howto_benchmark_workflow.md

[Gnurro]

I'm working on consolidating the backend/Model adding docs now.

[Gnurro]

https://github.com/clp-research/clembench/pull/64