Groups' Activity page is not documented

Problem to solve

The Groups' Activity page is not documented. Users seeking help do not have anywhere they can turn to for additional information.

!18869 (merged) added visibility of Epic events, but the docs could not be updated to match because there are no existing docs.

Proposal

Document the Groups' Activity page.

To view a Group's Activity page, go to a group and in the left sidebar, click Activity.