Update Group Security Dashboard documentation to recommend running a regular pipeline
In https://docs.gitlab.com/ee/user/group/security_dashboard/ we describe how the dashboard works, but we should also mention how to keep results up to date.
Since the list and the chart have different selection criteria (see https://gitlab.com/gitlab-org/gitlab-ee/issues/8886), the best way to keep everything consistent is to run a security pipeline at least daily.
We should add this suggestion to the documentation.