[DOCS] Adds intro for OIDC realm (#50485)

x-pack/docs/en/security/authentication/index.asciidoc

@@ -9,6 +9,7 @@ include::active-directory-realm.asciidoc[]
 include::file-realm.asciidoc[]
 include::ldap-realm.asciidoc[]
 include::native-realm.asciidoc[]
+include::oidc-realm.asciidoc[]
 include::pki-realm.asciidoc[]
 include::saml-realm.asciidoc[]
 include::kerberos-realm.asciidoc[]

x-pack/docs/en/security/authentication/oidc-realm.asciidoc

@@ -0,0 +1,19 @@
+[role="xpack"]
+[[oidc-realm]]
+=== OpenID Connect authentication
+
+The OpenID Connect realm enables {es} to serve as an OpenID Connect Relying
+Party (RP) and provides single sign-on (SSO) support in {kib}. 
+
+It is specifically designed to support authentication via an interactive web
+browser, so it does not operate as a standard authentication realm. Instead,
+there are {kib} and {es} {security-features} that work together to enable
+interactive OpenID Connect sessions.
+
+This means that the OpenID Connect realm is not suitable for use by standard
+REST clients. If you configure an OpenID Connect realm for use in {kib}, you
+should also configure another realm, such as the <<native-realm, native realm>>
+in your authentication chain.
+
+In order to simplify the process of configuring OpenID Connect authentication
+within the {stack}, there is a step-by-step guide: <<oidc-guide>>.