Add Milestone 18 to ESS builds (#646)

Looks like there are some items that cropped up that warrant a Milestone 18 doc build after all. So, as a first step, let's add a build and then we can haggle about ze details. 😸

Author: Nik Richers

conf.yaml

@@ -430,7 +430,7 @@ contents:
             tags:       Cloud/Reference
             subject:    Elastic Cloud
             current:    release-ms-17
-            branches:   [ release-ms-17, saas-release, saas-release-heroku ]
+            branches:   [ release-ms-18, release-ms-17, saas-release, saas-release-heroku ]
             index:      docs/saas/index.asciidoc
             chunk:      1
             private:    1