Question posed by a user recently:
This is not a function of the website, but you can do it using your browser app, at least on iOS (iPhone, iPad). On iOS, go to the site in your browser app (works in Chrome and Safari; I haven't tested any other browsers), then open the Share menu (icon is a little box with an up arrow on top), select "Add to Home Screen," and "Save." (The Share menu is in the top corner in Chrome; on the bottom toolbar in Safari.)
For me at least, if I do this in Safari, the "app" it creates is just a bookmark that opens Safari to the bookmarked page. In Chrome, while it's still technically a bookmark, the "app" it creates opens the bookmarked page as if it were a standalone app - the top Chrome navigation bar and icons are suppressed.
I don't use any Android devices, so can't comment on that - although Chrome tends to behave the same way cross-platform, so it may well support this function.
Any Android users care to chime in?