add initial smoke tests and documentation

doc/development/testing_guide/smoke.md

doc/development/testing_guide/testing_levels.md

qa/README.md

qa/qa.rb

qa/qa/scenario/taggable.rb

qa/qa/scenario/template.rb

qa/qa/scenario/test/instance.rb

qa/qa/scenario/test/instance/all.rb

qa/qa/scenario/test/instance/smoke.rb

qa/qa/specs/features/api/basics_spec.rb

qa/qa/specs/features/api/users_spec.rb

qa/qa/specs/features/login/basic_spec.rb

qa/qa/specs/features/login/ldap_spec.rb

qa/qa/specs/features/mattermost/group_create_spec.rb

qa/qa/specs/features/mattermost/login_spec.rb

qa/qa/specs/features/merge_request/create_spec.rb

qa/qa/specs/features/merge_request/rebase_spec.rb

qa/qa/specs/features/merge_request/squash_spec.rb

qa/qa/specs/features/project/activity_spec.rb

qa/qa/specs/features/project/add_deploy_key_spec.rb