Add Keycloak SAML to the documentation

[VincentSC]

Hi, I just got finished with https://www.linkedin.com/pulse/redash-login-via-keycloak-saml-vincent-hindriksen/ to explain how to get Redash working with Keycloak.

Would it be a good idea to add this to the documentation? If yes, I can rewrite it into the style of that page.

[arikfr]

This is not related to the issue, but I assume that the issue author or followers might have SAML enabled for their deployment and should be aware of the following Security Advisory: #5961. This affects all Redash versions and should be patched immediately.

[arikfr]

And to answer your question: sure! This will be much appreciated. Thanks.