Add missing CI files to global nav

As per the linked issue, ensures all CI/CD docs are in the global nav.

Resolves #967 (closed)

Latest list is below. Note that many were already in the nav:

• https://docs.gitlab.com/ee/administration/external_pipeline_validation.html
• https://docs.gitlab.com/ee/administration/job_artifacts.html
• (already there) https://docs.gitlab.com/ee/api/instance_level_ci_variables.html
• (already there) https://docs.gitlab.com/ee/ci/ci_cd_for_external_repos/index.html
• (already there) https://docs.gitlab.com/ee/ci/docker/README.html
• (already there) https://docs.gitlab.com/ee/ci/examples/end_to_end_testing_webdriverio/index.html
• (already there) https://docs.gitlab.com/ee/ci/examples/laravel_with_gitlab_and_envoy/index.html
• https://docs.gitlab.com/ee/ci/git_submodules.html
• (already there) https://docs.gitlab.com/ee/ci/merge_request_pipelines/pipelines_for_merged_results/index.html
• (already there) https://docs.gitlab.com/ee/ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/index.html
• (already there) https://docs.gitlab.com/ee/ci/ssh_keys/README.html