Rename final index files for compatibility with Hugo publishing

.gitlab/ci/rules.gitlab-ci.yml

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

doc/_index.md

doc/administration/get_started.md

doc/development/documentation/help.md

...development/documentation/site_architecture/automation.md

doc/legal/_index.md

doc/security/hardening_cicd_recommendations.md

doc/user/markdown.md

doc/user/project/settings/import_export.md

scripts/verify-tff-mapping

...i_configuration_validation/shared_context_and_examples.rb

spec/frontend/super_sidebar/components/help_center_spec.js

spec/haml_lint/linter/documentation_links_spec.rb

tests.yml