Another example is to enable comments or discussions on your HTML pages. There are several possibilities, such as Disqus (https://disqus.com) or Hypothesis (https://hypothes.is). These services can be easily embedded in your HTML book via the includes option (see Section 5.5 for details).
--- Source: bookdown.org
Good example for integration can be found here: https://benmarwick.github.io/bookdown-ort/mods.html