Getting started - Better empty state for Activity tab

Description

We need to know what the activity tab is all about when we first land on the page. This feature is awesome, but we need to let new users know how awesome it is. Currently, the empty state for this page is literally blank.

Issue similar to gitlab-foss#20848 (moved)

Wireframe

Links / references

• Part of gitlab-foss#15632 (closed)
• Issue similar to gitlab-foss#20848 (moved)