Docs: make it clear that the Jenkins integration relies on an unmaintained plugin

Problem to solve

This problem affects on our Jenkins CI service integration and only that specific section of our documentation. That integration relies on the jenkinsci/gitlab-ci plugin that has been seeking a maintainer for ~2 years and isn't actively supported anymore as a result. Our documentation does not make this clear to potential users.

Proposal

The note section at the top of our documentation on this integration should include a warning to potential users that there may be issues with it due to it not being updated or maintained and that GitLab is unable to support issues with it. In essence this note should:

1. Make it more clear that this plugin originates from a third party.
2. That it is not actively supported both by its former maintainer and GitLab.