[doc] Post Login Flow is sparcely documented and would benefit from a section

I think a section describing the post login flow with:

• what it does (add addition authentication steps when using brokered external identity provider)
• why one might use it (for example, OTP or session limits or maybe there are better examples)
• an example demonstrating that you have to create a new auth flow

Currently, the usage of Post Login Flow is only documented with a single line:
https://access.redhat.com/documentation/en-us/red_hat_single_sign-on/7.6/html/server_administration_guide/identity_broker#identity_broker_first_login

" Post Login Flow - The authentication flow Red Hat Single Sign-On triggers when a user finishes logging in with the external identity provider. "