Docs: fix duplicate titles

doc/administration/external_database.md

doc/administration/raketasks/project_import_export.md

doc/articles/openshift_and_gitlab/index.md

doc/ci/services/docker-services.md

doc/container_registry/README.md

doc/customization/issue_closing.md

doc/install/docker.md

doc/install/openshift_and_gitlab/index.md

doc/legal/README.md

doc/legal/corporate_contributor_license_agreement.md

doc/legal/individual_contributor_license_agreement.md

doc/raketasks/features.md

doc/raketasks/web_hooks.md

doc/university/training/gitlab_flow.md

doc/university/training/topics/gitlab_flow.md

doc/university/training/topics/merge_requests.md

doc/user/admin_area/labels.md

doc/user/project/pages/introduction.md

doc/user/project/settings/import_export.md

doc/workflow/protected_branches.md