Sphinx configuration improvements

[benoit9126]

Small improvements of the Sphinx configuration:

• Delete the autodoc configuration part (we are using autoapi)
• Add some Intersphinx mappings
• Delete the warning for the roseau.load_flow_engine.cy_engine module which is not found when generating the documentation.

@alihamdan By the way, the command make linkcheck already exists and checks all the external links found in the documentation. We should run it sometimes to ensure the documentation is up-to-date.

[benoit9126]

There is still a bug in sphinx-autoapi readthedocs/sphinx-autoapi#473 that needs to be solved to have a better documentation (ie. API doc similar to the "API Reference" section for each model)