add broken link hrefs for ee and gitlab-versions

doc/ci/yaml/README.md

@@ -2031,3 +2031,5 @@ CI with various languages.
 [ce-12909]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12909
 [schedules]: ../../user/project/pipelines/schedules.md
 [variables-expressions]: ../variables/README.md#variables-expressions
+[ee]: https://about.gitlab.com/gitlab-ee/
+[gitlab-versions]: https://about.gitlab.com/products/
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