Add single sign on documentation

[martinlagler]

Q	A
Related PRs	sulu/sulu#7262
License	MIT

What's in this PR?

Documentation for Single Sign on configuration.

Why?

To document the new feature

[Kurt108]

Is it possible to redirect every user to the IAM-system - no matter which e-mail they are entering for login?

[alexander-schranz]

@Kurt108 not yet but think we could add support for something like a * domain it should be quiet easy to contribute if you want just something like this and add some tests to it:

[Kurt108]

alright. I will upgrade our current installation and hope our developer can provide the contribution.