Are there tools for annotating unfamiliar codebases?

I often finding myself trying to parse through a big repo. Often a project's API is well documented but its internal architecture isn't.

Ideally I'd like to be able to select lines or a file in someone else's GitHub repository, add an annotation, then zoom out and look at an outline.

Even better if I could automatically make these notes public as a form of documentation to make life easier for the poor soul after me.

Does something like this exist? Hoping some developer is just terrible at marketing.