Getting started - Better empty state for Merge requests view

Description

This issue is similar to #20854 (closed). For users truly new to Git, we need to explain what merge requests are and how they can improve the life of a team. Currently the empty state for the page listing all merge request is literally a blank page.

Wireframe

Links / references

• Part of #15632 (closed)
• Issue similar to #20854 (closed)