Add example to plugins file
What does this MR do?
Add an example to the plugins page that uses a little bit more of the plugin feature.
I created the merge request based on @dblessing issue shared below.
Related issues
https://gitlab.com/gitlab-org/gitlab-ce/issues/61120
Author's checklist
-
Follow the Documentation Guidelines and Style Guide. -
Link docs to and from the higher-level index page, plus other related docs where helpful. -
Apply the ~Documentation label.