Readmes and most docs on GitHub are fairly linear, a normal site with navigation menu is IMHO easier to navigate. I want to be able to search docs once they are longer than a single page.
What I mean with linear:
With the repo you linked, I have to scroll past the file list, then the readme-header, then your twitter links, then the gif to even get to the introduction which tells me what I'm looking at. Next is installation, which has to be in the readme, but I don't care right now because I still want to see more details. With e.g. a navigation bar on the side I could have simply clicked on "Introduction" and then "quickstart" to get what I want.
And all GitHub repos look the same, so I won't recognize the site by it's design if I find it again a while later. Nothing memorable there.
That doesn't mean that a site should be a high priority for you, but these are some reasons why you might want one.
