Multiple confusing documentations for subscriptions

I'm trying to understand how the following three heavily overlapping docs relate to each other:

1. [this `apollographql/graphql-subscriptions` README](https://github.com/apollographql/graphql-subscriptions)
2. this ["graphql-subscriptions" guide](https://www.apollographql.com/docs/graphql-subscriptions/subscriptions-to-schema/)
* note that the "Edit on GitHub" link on that page points to a different repo, not this one, even though this repo *is* "graphql-subscriptions"
3. https://www.apollographql.com/docs/apollo-server/data/subscriptions/

Compare the repository README with the linked graphql-subscriptions and Apollo Server subscription guides. Start by tracing the documentation ownership, including the incorrect “Edit on GitHub” destination. Done means the overlap and relationship between the three documents are clearly explained and the repository’s documentation links to the appropriate source.

Written by the indexing model from the issue text.