Be notified when people annotate a page or site

### Problem you are trying to address

As a publisher of content on the web
I want to be notified when people make public annotations on my content
So that I can see the conversations that my content has inspired
And so that I can respond, if appropriate

As a user who has annotated a web page
I want to be notified when people make public annotations on the same page
So that I can continue to engage in discussions around that topic

As a member of an online community
I want to be notified when people make public annotations on that community's web pages
So that I can engage with them and be an active member of the community

### User requests

- https://hypothesis.zendesk.com/agent/tickets/201
- https://hypothesis.zendesk.com/agent/tickets/350
- https://hypothesis.zendesk.com/agent/tickets/894
- https://hypothesis.zendesk.com/agent/tickets/988
- https://hypothesis.zendesk.com/agent/tickets/1000

Start by reviewing the five linked Zendesk requests and the user stories in this issue. No source files, tests, or entry points are identified, so the notification scope and acceptance criteria need to be established before implementation; done should cover the agreed notification behavior for publishers, annotators, and community members.

Written by the indexing model from the issue text.