you are right, there is no technical instruction on how to do this, but if you look at the source of the website, you can find it how to do embed an app.
This is a simplified version of how the google trend index app is integrated on the RStudio website
<html>
<head><title>Shiny App Iframe</title></head>
<body>
<iframe id="example1" src="https://gallery.shinyapps.io/goog-trend-index/" style="border: none; width: 100%; height: 850px" frameborder="0"></iframe>
</body>
</html>