Merge pull request #25901 from yuexiao-wang/add-content

Add the content for how services work
(cherry picked from commit abd08f4784810bcf8b4e88e3f9ef1023cb15b8b5)

Signed-off-by: Charles Smith <charles.smith@docker.com>

docs/swarm/how-swarm-mode-works/menu.md

@@ -15,3 +15,4 @@ weight=11
 ## TOC
 
 * [How nodes work](nodes.md)
+* [How services work](services.md)