Document basic agent connection details

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/125299



Merged-by: Phillip Wells <pwells@gitlab.com>
Approved-by: Mikhail Mazurskiy <mmazurskiy@gitlab.com>
Approved-by: Phillip Wells <pwells@gitlab.com>
Reviewed-by: Viktor Nagy (GitLab) <vnagy@gitlab.com>
Reviewed-by: Mikhail Mazurskiy <mmazurskiy@gitlab.com>
Reviewed-by: Phillip Wells <pwells@gitlab.com>
Co-authored-by: Timo Furrer <tfurrer@gitlab.com>