Merge branch 'docs-add-jenkins-integrations-page' into 'master'

Update project_services.md

Closes #3253

See merge request !2767

doc/user/project/integrations/project_services.md

@@ -38,6 +38,7 @@ Click on the service links to see further configuration instructions and details
 | [HipChat](hipchat.md) | Private group chat and IM |
 | [Irker (IRC gateway)](irker.md) | Send IRC messages, on update, to a list of recipients through an Irker gateway |
 | [JIRA](jira.md) | JIRA issue tracker |
+| [Jenkins](../../../integration/jenkins.md) | An extendable open source continuous integration server |
 | JetBrains TeamCity CI | A continuous integration and build server |
 | [Kubernetes](kubernetes.md) | A containerized deployment service |
 | [Mattermost slash commands](mattermost_slash_commands.md) | Mattermost chat and ChatOps slash commands |