[unimpressive]

You want https://hypothes.is

You're going to get a lot of dismissal here because the idea is old and has been tried many times. I wouldn't let that stop you, annotation is very useful. The thing I am skeptical of however is that it makes the most sense 'as a forum'. I don't think it makes a very good forum, it does make an excellent collaborative research tool. More sane would be to have these annotations and import them into a real forum tool.