Create docstrings for all public functions and create OpenFL API Documentation

[psfoley]

Most of OpenFL functions are documented already, but this task involves making sure that they are all complete and up to date. After that is done, suggest using sphinx-apidoc to automatically generate API documentation and this to the OpenFL documentation.

[manuelhsantana]

PR created: https://github.com/securefederatedai/openfl/pull/900

[MasterSkepticista]

Merged #1005