Add documentation page stubs for Single Sign-On, SAML and CAS pages (#11298)

Author: anoadragon453

changelog.d/11298.doc

@@ -0,0 +1 @@
+Add Single Sign-On, SAML and CAS pages to the documentation.

docs/SUMMARY.md

@@ -23,10 +23,10 @@
     - [Structured Logging](structured_logging.md)
     - [Templates](templates.md)
     - [User Authentication](usage/configuration/user_authentication/README.md)
-      - [Single-Sign On]()
+      - [Single-Sign On](usage/configuration/user_authentication/single_sign_on/README.md)
         - [OpenID Connect](openid.md)
-        - [SAML]()
-        - [CAS]()
+        - [SAML](usage/configuration/user_authentication/single_sign_on/saml.md)
+        - [CAS](usage/configuration/user_authentication/single_sign_on/cas.md)
         - [SSO Mapping Providers](sso_mapping_providers.md)
       - [Password Auth Providers](password_auth_providers.md)
       - [JSON Web Tokens](jwt.md)

docs/usage/configuration/user_authentication/single_sign_on/README.md

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+# Single Sign-On
+
+Synapse supports single sign-on through the SAML, Open ID Connect or CAS protocols. 
+LDAP and other login methods are supported through first and third-party password
+auth provider modules.

docs/usage/configuration/user_authentication/single_sign_on/cas.md

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+# CAS
+
+Synapse supports authenticating users via the [Central Authentication
+Service protocol](https://en.wikipedia.org/wiki/Central_Authentication_Service)
+(CAS) natively.
+
+Please see the `cas_config` and `sso` sections of the [Synapse configuration
+file](../../../configuration/homeserver_sample_config.md) for more details.

docs/usage/configuration/user_authentication/single_sign_on/saml.md

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+# SAML
+
+Synapse supports authenticating users via the [Security Assertion
+Markup Language](https://en.wikipedia.org/wiki/Security_Assertion_Markup_Language)
+(SAML) protocol natively.
+
+Please see the `saml2_config` and `sso` sections of the [Synapse configuration
+file](../../../configuration/homeserver_sample_config.md) for more details.