DAST CI documentation is not reflecting the need for docker-in-docker

As we introduced vendored templates, we updated the documentation accordingly. But this doc doesn't reflect a critical change: Docker-in-Docker is now required.

Current prerequisites: https://docs.gitlab.com/ee/ci/examples/dast.html#prerequisites usage of docker in docker: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/lib/gitlab/ci/templates/Security/DAST.gitlab-ci.yml#L28

We should make sure with @bikebilly that this change is fine: Requiring dind is a big impacting change for users.

/cc @twoodham @vzagorodny