Docs update

[kkaris]

This PR updates the documentation in the repository to be more complete, this includes docstrings, file headers and REST API documentation.

A docs build for this branch is set up here: https://miramodel.readthedocs.io/en/docs-update/index.html

Closes #264

[kkaris]

The schema test erred because it was generated locally using pydantic 1.10.12, while the latest pydantic<2 is 1.10.13.

[kkaris]

(copied comment from issue)
A bunch of classes are excluded from the docs for the templates module, mira.metamodel.templates, see docs/source/metamodel.rst:

Templates (:py:mod:`mira.metamodel.templates`)
----------------------------------------------
.. automodule:: mira.metamodel.templates
    :members:
    :exclude-members: Concept, ControlledConversion, NaturalConversion, Provenance, Template, NaturalDegradation, NaturalProduction, GroupedControlledConversion
    :show-inheritance:

Should we include them now that we anticipate automated reading of the documentation?