Epics relantioship API

What does this MR do?

Add API endpoints for listing, adding and removing related epics.

What are the relevant issue numbers?

closes #7326 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

doc/api/README.md

@@ -23,6 +23,7 @@ The following API resources are available:
 - [Discussions](discussions.md) (threaded comments)
 - [Environments](environments.md)
 - [Epic issues](epic_issues.md) **[ULTIMATE]**
+- [Epic links](epic_links.md) **[ULTIMATE]**
 - [Epics](epics.md) **[ULTIMATE]**
 - [Events](events.md)
 - [Feature flags](features.md)