Add @UpdateForV9 for ReferenceDocs (#123928)

The docs system is very different in v9 so we must update
`ReferenceDocs` to generate links that are not broken.

libs/core/src/main/java/org/elasticsearch/core/UpdateForV9.java

@@ -25,6 +25,7 @@ public @interface UpdateForV9 {
         CORE_INFRA,
         DATA_MANAGEMENT,
         DISTRIBUTED_INDEXING,
+        DOCS,
         ENTERPRISE_SEARCH,
         MACHINE_LEARNING,
         PROFILING,