How to customize the first page when someone comes to my Mattermost site?

This is the page I am talking about:

I want to change it so that it reads something like this…

Welcome our virtual office! We build with Mattermost which offers an app you can use to access our site or click the link at the very bottom to use it as a web app.

Strange, I searched deep and wide, no one seems to ask this question.

So no one knows how?

This document may help: Customizing Mattermost — Mattermost documentation.