Update env-vars-settings document in admin-guide: support for variables related to static login

[ism-k]

Background

Created in response to a comment in this thread:
getredash/contrib-helm-chart#122

Problem

The current list in the Administrator's Guide does not list any environment variables for SAML static login.
I find REDASH_SAML_SSO_URL to be particularly important as it is a required configuration item to enable SAML login in Redash v10.

Change

• Added new variables to the list of environment variables in the Administrator's Guide:
  https://redash.io/help/open-source/admin-guide/env-vars-settings

The user-guide has a related description, so I have tried to correspond with this one:

website/src/pages/kb/user-guide/users/authentication-options.md

Lines 90 to 96 in fa2f93b

	### Static SAML
	Static configuration requires these fields:
	- **SAML Single Sign-on URL** is the URL at your IDP where users will be redirected when they click the _SAML Login_ button in Redash.
	- **SAML Entity ID** should be the URL to your Redash instance.
	- **SAML x509 cert** will be provided by your IDP.