Add instructions on how to configure Auth0 to the docs

[arikfr]

@madAndroid was kind enough to provide instructions on how to configure Redash with Auth0/SAML on the forum: https://discuss.redash.io/t/auth0-integration/586/5.

We should port this over to the Knowledge Base (either as a section in the Authentication Options page or a dedicated one).

[r0cketr1kky]

Check out this pr: getredash/website#336

[spicypenguin]

The related PR above has been merged, can this issue be closed now?

[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.