Add API documentation

[Phil-DS]

master

Code Version

master

Expected Behavior

Find some API docs for pysaml2

Current Behavior

There are no API docs. Just a fairly complicated example, some tests, and a fairly in-depth config documentation/setup the examples.

Possible Solution

Creating API docs that allow a user to understand what there is to offer in the module.

Steps to Reproduce

1. Go to https://pysaml2.readthedocs.io/en/latest/index.html
2. Look for API documentation
3. Can't find API documentation
4. Sift through lengthy example to try figure out what is happening

This is similar to #689, except, instead of just minimal examples, I was trying to find API docs to see what functionality there is, to try to even understand the examples.