Adding docs for how to configure Okta for the OIDC provider (#235)

* Adding documentation for Okta OIDC provider.

* additional clean up.

* Clearer heading

* Forgot a word.

* updated documentation based on ReillyProcentive review.

* Per steakunderscore review: removed defaults. Removed extra hardening steps (expiration, https only etc) not directly related to setting up Okta w/ OIDC