Add a link in the docs to the epic that defined the Insights feature

What does this MR do?

This fixes the "Introduced in GitLab 12.0" message for the Insights feature.

I chose to remove the mention of the feature flag since it's not super helpful anyway, we care about the General Availability date.

Follow-up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29494/diffs.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #63463 (closed).