Add list related epic links API documentation

Add API documentation for related epic links GET
endpoint.

Changelog: other

doc/api/api_resources.md

@@ -118,6 +118,7 @@ The following API resources are available in the group context:
 | [Invitations](invitations.md)                                    | `/groups/:id/invitations` (also available for projects) |
 | [Issues](issues.md)                                              | `/groups/:id/issues` (also available for projects and standalone) |
 | [Issues Statistics](issues_statistics.md)                        | `/groups/:id/issues_statistics` (also available for projects and standalone) |
+| [Linked epics](linked_epics.md)                  | `/groups/:id/epics/.../related_epics` |
 | [Members](members.md)                                            | `/groups/:id/members` (also available for projects) |
 | [Merge requests](merge_requests.md)                              | `/groups/:id/merge_requests` (also available for projects and standalone) |
 | [Notes](notes.md) (comments)                                     | `/groups/:id/epics/.../notes` (also available for projects) |

doc/api/linked_epics.md

+---
+stage: Plan
+group: Product Planning
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Linked epics API **(ULTIMATE)**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/352493) in GitLab 14.9 [with a flag](../administration/feature_flags.md) named `related_epics_widget`. Disabled by default.
+
+FLAG:
+On self-managed GitLab, by default this feature is not available. To make it available, ask an administrator to [enable the feature flag](../administration/feature_flags.md) named `related_epics_widget`. On GitLab.com, this feature is not available.
+
+If the Related Epics feature is not available in your GitLab plan, a `403` status code is returned.
+
+## List linked epics
+
+Get a list of a given epic's linked epics filtered according to the user authorizations.
+
+```plaintext
+GET /groups/:id/epics/:epic_iid/related_epics
+```
+
+Supported attributes:
+
+| Attribute  | Type           | Required               | Description                                                               |
+| ---------- | -------------- | ---------------------- | ------------------------------------------------------------------------- |
+| `epic_iid` | integer        | **{check-circle}** Yes | Internal ID of a group's epic                                             |
+| `id`       | integer/string | **{check-circle}** Yes | ID or [URL-encoded path of the group](index.md#namespaced-path-encoding). |
+
+Example request:
+
+```shell
+curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/groups/:id/epics/:epic_iid/related_epics"
+```
+
+Example response:
+
+```json
+[
+   {
+      "id":2,
+      "iid":2,
+      "color":"#1068bf",
+      "text_color":"#FFFFFF",
+      "group_id":2,
+      "parent_id":null,
+      "parent_iid":null,
+      "title":"My title 2",
+      "description":null,
+      "confidential":false,
+      "author":{
+         "id":3,
+         "username":"user3",
+         "name":"Sidney Jones4",
+         "state":"active",
+         "avatar_url":"https://www.gravatar.com/avatar/82797019f038ab535a84c6591e7bc936?s=80u0026d=identicon",
+         "web_url":"http://localhost/user3"
+      },
+      "start_date":null,
+      "end_date":null,
+      "due_date":null,
+      "state":"opened",
+      "web_url":"http://localhost/groups/group1/-/epics/2",
+      "references":{
+         "short":"u00262",
+         "relative":"u00262",
+         "full":"group1u00262"
+      },
+      "created_at":"2022-03-10T18:35:24.479Z",
+      "updated_at":"2022-03-10T18:35:24.479Z",
+      "closed_at":null,
+      "labels":[
+
+      ],
+      "upvotes":0,
+      "downvotes":0,
+      "_links":{
+         "self":"http://localhost/api/v4/groups/2/epics/2",
+         "epic_issues":"http://localhost/api/v4/groups/2/epics/2/issues",
+         "group":"http://localhost/api/v4/groups/2",
+         "parent":null
+      },
+      "related_epic_link_id":1,
+      "link_type":"relates_to",
+      "link_created_at":"2022-03-10T18:35:24.496+00:00",
+      "link_updated_at":"2022-03-10T18:35:24.496+00:00"
+   }
+]
+```