docs: Add OpenID Connect to the Authentication concepts page

Author: siegfriedweber

modules/concepts/examples/authenticationclass-keycloak.yaml

@@ -0,0 +1,16 @@
+apiVersion: authentication.stackable.tech/v1alpha1
+kind: AuthenticationClass
+metadata:
+  name: keycloak
+spec:
+  provider:
+    oidc:
+      hostname: my.keycloak.server        # <1>
+      port: 8080                          # <2>
+      rootPath: /realms/master            # <3>
+      scopes:                             # <4>
+      - email
+      - openid
+      - profile
+      principalClaim: preferred_username  # <5>
+      providerHint: Keycloak              # <6>

modules/concepts/pages/authentication.adoc

@@ -14,6 +14,7 @@ Multiple operators use this CRD as a way to express and configure the authentica
 The following authentication providers are supported:
 
 * <<LDAP>>: Authenticate users using an LDAP server.
+* <<OIDC>>: Authenticate users using an OpenID connect provider.
 * <<TLS>>: Authenticate users with client TLS certificates.
 * <<Static>>: Authenticate users against a static list of users and passwords in a simple Kubernetes Secret.
 
@@ -36,6 +37,25 @@ image::image$authentication-overview.drawio.svg[]
 
 NOTE: Learn more in the xref:tutorials:authentication_with_openldap.adoc[OpenLDAP tutorial] and get a full overview of all the properties in the {crd-docs}/authentication.stackable.tech/authenticationclass/v1alpha1/#spec-provider-ldap[AuthenticationClass LDAP provider CRD reference].
 
+[#OIDC]
+=== OpenID Connect
+
+An OIDC provider like https://www.keycloak.org/[Keycloak {external-link-icon}^] could be configured as follows:
+
+[source,yaml]
+----
+include::example$authenticationclass-keycloak.yaml[]
+----
+
+<1> Hostname of the identity provider.
+<2> Port of the identity provider. If TLS is used defaults to 443, otherwise to 80.
+<3> Root HTTP path of the identity provider. Defaults to `/`.
+<4> Scopes to request from your identity provider. It is recommended to request the `openid`, `email`, and `profile` scopes.
+<5> If a product extracts some sort of "effective user" that is represented by a string internally, this config determines which claim is used to extract that string.
+<6> This is a hint about which identity provider is used by the AuthenticationClass.
+
+NOTE: Get a full overview of all the properties in the {crd-docs}/authentication.stackable.tech/authenticationclass/v1alpha1/#spec-provider-oidc[AuthenticationClass OIDC provider CRD reference].
+
 [#tls]
 === TLS
 The `TLS` provider configures a product to authenticate users using TLS certificates.