The add-on called "UNDER CONSTRUCTION"Browser Info Environment
My situation is that I have already deleted my old 5.6 website, and am now trying to quickly create a new 5.7 website. During the days before my 5.7 website will be installed, I would like to have something online, just in case a visitor comes to my website.
It does sound like this is the kind of thing you might be looking for. You want to create a new page where you would display your information about the site while you are setting it up. For example you might create a page entitled "Coming Soon". To enable that you would perform two steps.
The first step is to go to the "Under Construction" settings dashboard page and select your new "Coming Soon" page.
The second would be to go to the concrete5 built in permissions system to assign a user or group permission to bypass the redirect. So for example you have the "admin" user. You could assign the "admin" user the permission to see the entire site and then all other visitors would be redirected to your Coming Soon page. Or you could have the group "Editors" and assign that group permission to bypass the redirect. Then add any users to that group.
Hopefully the expands a bit more on the documentation. Let me know if you are having any issues. Its always possible there may be a bug you are experiencing.