Hedgedoc is a web-based, collaborative markdown editor, hosted on the hedgedoc.cats.sh subdomain.
Press either "Sign In" buttons, then press "Sign in via auth.cats.sh", then login as you normally would to
auth.cats.sh with the username and password you registered with. You will be redirected back to Hedgedoc, now logged in.
Hedgedoc doas provide a Documentation site as well as a Frequently Asked Questions page, though they are more focused on the hosting side.