[Docs] Have mergify leave a comment with the docs preview link (#26412)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

.github/mergify.yml

@@ -11,6 +11,8 @@ pull_request_rules:
     label:
       add:
         - documentation
+    comment:
+      message: "Documentation preview: https://vllm--{{number}}.org.readthedocs.build/en/{{number}}/"
 
 - name: label-ci-build
   description: Automatically apply ci/build label