Jenkins documentation should be clearer

Report https://twitter.com/Nitrodist/status/700758609974546433

In CE people can use:

1. Commit status API https://gitlab.com/gitlab-org/gitlab-ce/issues/2595
2. GitLab plugin https://wiki.jenkins-ci.org/display/JENKINS/GitLab+Plugin

In EE people also get:

1. GitLab project service http://doc.gitlab.com/ee/integration/jenkins.html

Changes we should make:

1. Make it clear on http://doc.gitlab.com/ee/integration/jenkins.html that they can use Jenkins without the project service if they want.
2. Make it clear what they gain if they use the GitLab project service.
3. Add link to http://doc.gitlab.com/ee/integration/jenkins.html from https://about.gitlab.com/applications/ in the section CI services

Comments are welcome.

/cc @axil @marin