Add docs navigation entry for pipeline execution policy page

Add navigation item to global navigation

• Link to the merge request that introduced the new page: Add documentation for pipeline execution policies (gitlab!157422 - merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://rd-add-pipeline-execution-policy.docs.gitlab-review.app/ee/user/application_security/policies/pipeline_execution_policies.html