I have a markdown (MD) page on GitHub in a public repository that I want to add some additional information when someone clicks on a hyperlink. Ideally something similair to Bootstrap's Popover.
Is this possible and if so how?
Not directly with GitHub page, which only support GFM (GitHub Flavored Markdown Spec)
What you can do is use your GitHub repo with those mardown pages as sources for generating a static website (where you can add any additional information to the HTML code you are generating).
A static website generator like Hugo can help.
See "Using a static site generator other than Jekyll" for the process.