docs: add ReadBody route predicate factory documentation

Adds documentation for ReadBodyRoutePredicateFactory which was missing
from the request predicate factories reference guide.

Documents Java DSL usage and explicitly notes that YAML configuration
is not supported since the predicate parameter requires a Java lambda.

Fixes #3569

Signed-off-by: pachat <pacifiquepachat.hategekimana@risa.gov.rw>

Author: pachat

docs/modules/ROOT/pages/spring-cloud-gateway-server-webflux/request-predicates-factories.adoc

@@ -376,3 +376,40 @@ spring:
 
 This route matches if the `X-Forwarded-For` header contains, for example, `192.168.1.10`.
 
+
+[[read-body-route-predicate-factory]]
+== The ReadBody Route Predicate Factory
+
+The `ReadBody` route predicate factory reads the request body and applies a user-provided
+predicate against it. The body is cached in memory after the first read, so subsequent
+evaluations do not need to deserialize it again.
+
+NOTE: This predicate can only be configured programmatically via the Java DSL.
+It cannot be configured via YAML because the `predicate` parameter requires a Java
+`Predicate` lambda, which cannot be expressed in a configuration file.
+
+The following example configures a ReadBody route predicate:
+
+.GatewayConfig.java
+[source,java]
+----
+@Bean
+public RouteLocator routes(RouteLocatorBuilder builder) {
+    return builder.routes()
+        .route("read-body-route", r -> r
+            .readBody(String.class, body -> body.contains("yes"))
+            .uri("https://example.org"))
+        .build();
+}
+----
+
+In the preceding example, the predicate checks whether the request body (deserialized as
+a `String`) contains the word `yes`. If so, the route matches and the request is forwarded
+to `https://example.org`.
+
+The `readBody` method accepts two parameters:
+
+* `inClass` - the class to deserialize the request body into (for example, `String.class`
+or a custom request DTO class).
+* `predicate` - a Java `Predicate<T>` that receives the deserialized body and returns
+`true` if the route should match.