Add WebFlux OAuth2 Client Reference

Fixes: gh- 5865

docs/manual/src/docs/asciidoc/_includes/reactive/oauth2/access-token.adoc

@@ -0,0 +1,34 @@
+= Access Token
+
+Spring Security's OAuth Support allows obtaining an access token without authenticating.
+A basic configuration with Spring Boot can be seen below:
+
+[source,yml]
+----
+spring:
+  security:
+    oauth2:
+      client:
+        registration:
+          github:
+            client-id: replace-with-client-id
+            client-secret: replace-with-client-secret
+            scopes: read:user,public_repo
+----
+
+You will need to replace the `client-id` and `client-secret` with values registered with GitHub.
+
+The next step is to instruct Spring Security that you wish to act as an OAuth2 Client so that you can obtain an access token.
+
+[source,java]
+----
+@Bean
+SecurityWebFilterChain configure(ServerHttpSecurity http) throws Exception {
+	http
+		// ...
+		.oauth2Client();
+	return http.build();
+}
+----
+
+You can now leverage Spring Security's <<webclient>> support to obtain and use the access token.

docs/manual/src/docs/asciidoc/_includes/reactive/oauth2/index.adoc

@@ -3,3 +3,5 @@
 Spring Security provides OAuth2 and WebFlux integration for reactive applications.
 
 include::login.adoc[leveloffset+=1]
+
+include::access-token.adoc[leveloffset+=1]