TL;DR Recently I have switched from Jekyll to Docusaurus for my personal website. And immediately noticed the issue with letting the visitors place comments to blog posts. It seems that the comments work out of the box only for Facebook comments and then only if one has a registered FB business site (according to this PR). Searching on adding e.g. Disqus blog comments in Docusaurus brought back more questions than answers. (see some references at the end of this post)
So, I have made my own go on this issue and finally succeeded in adding the Disqus blog comments to my Docusaurus blog. In this blog post you can see them in action and also read how I did it. I hope it might help some of you to do the same on yours.