Update documentation for Epics filtered search and Group Roadmap

What does this MR do?

Adds documentation for following two MRs;

• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4223: Filtered search support for Epics list page
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4361: Show Group level Roadmap

Links to documentation

• /help/user/group/epics/index.md
• /help/user/group/roadmap/index.md

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• Documentation created/updated
• [ ] API support added
• [ ] Tests added for this feature/bug
• Review
  • Has been reviewed by Documentation
  • [ ] Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [ ] Internationalization required/considered
• [ ] If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• [ ] End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/4656

https://gitlab.com/gitlab-org/gitlab-ee/issues/3559