Rename index.md to _index.md for compatibility with docs-gitlab-com

app/assets/javascripts/gitlab_version_check/constants.js

...sets/javascripts/super_sidebar/components/help_center.vue

...roups/_import_group_from_another_instance_panel.html.haml

app/views/projects/triggers/_index.html.haml

doc/administration/backup_restore/restore_gitlab.md

doc/administration/compliance.md

...administration/geo/replication/upgrading_the_geo_sites.md

doc/administration/get_started.md

doc/administration/git_protocol.md

doc/administration/gitaly/configure_gitaly.md

doc/administration/gitaly/troubleshooting.md

doc/administration/index.md

doc/administration/license.md

doc/administration/license_file.md

doc/administration/monitoring/prometheus/web_exporter.md

doc/administration/package_information/omnibus_packages.md

doc/administration/package_information/supported_os.md

doc/administration/pages/index.md

doc/administration/reference_architectures/1k_users.md

doc/administration/reference_architectures/index.md