Consider regenerating the api stubs

[diego-plan9]

Description and motivation

While revising the readthedocs integration (#576), a number of slight discrepancies between the apidocs structure and the current module structure was found. It would be great to regenerate the docs/source/api/ folder, to ensure the rendered documentation contains the right set of entries for the modules.

Proposed solution

Refresh the contents of the folder via:

• installing the package (to avoid rpu_base stub deletion)
• remove the current files (rm -r docs/source/api/*.rst)
• regenerate the stubs (cd docs; make apidoc)
• sanity check on the rendered output, ensuring the content is correct

Alternatives and other information

• It might be needed to tweak the apidoc makefile target at https://github.com/IBM/aihwkit/blob/master/docs/Makefile#L22 in order to add extra files related to inference.
• Also, docs/source/api_reference.rst might need to be updated manually. Originally, the file only contained the second-level modules, mostly to avoid having to keep it manually updated and just be able to delegate onto docs/source/api/aiwhkit.rst (which is automatically regenerated via the make apidoc command) for the full information.