Expand and improve `Supported APIs` section in docs

It would be great to have a table that shows which APIs have distributed/parallel implementations for different execution backends (pandas_on_ray, pandas_on_dask, omnisci_on_native, etc.) including some notes when those partly are implemented and default to pandas. (current version is here - https://modin.readthedocs.io/en/stable/supported_apis/index.html)