DOC: use sphinx autoapi instead of autosummary

[SMoraisAnsys]

Description of the modifications

Pyansys project are switching from sphinx extension autosummary to autoapi.
This should probably wait a bit as we might need some changes associated to readthedocs/sphinx-autoapi#399. Since this PR is taking quiet some time to be merged, the project might be forked by pyansys and we should point to that version in that case.

Note: changing the extensions should solve the issue we are having when building pdf (using sphinx-build -M latex with numpydoc and autosummary). Currently we aren't able to remove the warnings that are triggered on the Enum classes.

Useful links and references

#354 (comment)