Mention HAProxy and OpenSearch docker variables in Offline mode

Expand Offline Setup Guidance with the guidance for HAProxy and OpenSearch docker variables.

For Offline setup, Ansible tasks rely on installation of specific docker images such as haproxy_docker_image and opensearch_docker_image. For customer following offline setup, documentation should be updated with the guidance which images should be pulled and prepared before running the toolkit if they plan to use HAProxy or OpenSearch. This section can be expanded with this information https://gitlab.com/gitlab-org/gitlab-environment-toolkit/-/blob/main/docs/environment_advanced_network.md#ansible-galaxy-roles