Add code documentation

[SchrodingersGat]

Body of the issue

Ref: #2493 (comment)

Ref: https://github.com/mkdocstrings/mkdocstrings

Build code documentation into the InvenTree docs automatically

[SchrodingersGat]

It might be an idea to add a decorator to functions we wish to specify as "API" functions (which we make an effort not to change), while some functions are marked as internal. Or mark internal functions with an underscore prefix, or similar.

Having an API of public functions would help to make them more discoverable for inclusion in generated reports, for example

[matmair]

@SchrodingersGat I think it makes sense to only expose things we want to be included specifically like plugin mixins.

[SchrodingersGat]

So you would suggest an "opt in" approach for which classes and methods we expose?

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[SchrodingersGat]

not stale

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[matmair]

Still important

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[matmair]

still important

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[wolflu05]

I think this is still relevant because this would reduce a lot of duplicate documentation in the docstrings and the documentation.

The suggested library was now also already added as part of #4824 documentation.