Docs: Add use cases for linked epics

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175157



Merged-by: Brendan Lynch <blynch@gitlab.com>
Approved-by: Brendan Lynch <blynch@gitlab.com>
Reviewed-by: Brendan Lynch <blynch@gitlab.com>
Co-authored-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Co-authored-by: Amanda Rueda <2890431-amandarueda@users.noreply.gitlab.com>