Merge branch 'nd/api-epic-link-documentation' into 'master'

Add Open API documentation for EpicLinks

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/105405



Merged-by: Doug Stull <dstull@gitlab.com>
Approved-by: Jon Jenkins <jjenkins@gitlab.com>
Approved-by: Doug Stull <dstull@gitlab.com>
Co-authored-by: Nicolas Dular <ndular@gitlab.com>