Docs: model_specification.rst

[ceholden]

Documentation for model parameter specification guide page docs/guide/model_specification.rst

https://jacobian.org/writing/great-documentation/

[ceholden]

Note the typo fix in the original issue -- this one is about model specification, not exploration. My intent on this was to describe the configuration file in general, with sub-sections about configuring the timeseries algorithms.

No super detailed low-level explanations about what each parameter means for CCDCesque, for example, just a brief description is necessary I think. In the future there should be sub-sections for other algorithms included in YATSM, but it would be a good start to begin with just CCDCesque.