HedgeDoc (formerly known as CodiMD, formerly known as HackMD) is a collaborative, web-based markdown editor (a.k.a a "pad").
Software Heritage hosts an instance of HedgeDoc on https://hedgedoc.softwareheritage.org/
A list of the HedgeDoc features is available as a self-hosted document on the instance. Most notably, compared to other pads, as the owner of a note (after you're logged in), you can configure whether guests can read or edit the note, or whether other logged in users can read or edit the note.
Hedgedoc is accessible to the public; Guests can (at least currently) create notes as well. Authentication is wired to the Software Heritage SSO service.
It is deployed using the profile::hedgedoc puppet manifest.