Change button text from "Create/add new role" to "Create role"

doc/development/permissions/custom_roles.md

doc/user/custom_roles.md

...sets/javascripts/roles_and_permissions/components/app.vue

...s/roles_and_permissions/components/create_member_role.vue

...s_and_permissions/components/custom_roles_empty_state.vue

...ts/roles_and_permissions/components/list_member_roles.vue

ee/spec/features/admin/member_roles_spec.rb

ee/spec/features/groups/member_roles_spec.rb

...pec/frontend/roles_and_permissions/components/app_spec.js

...d_permissions/components/custom_roles_empty_state_spec.js

locale/gitlab.pot