Add Subscriptions to Learn section

See: https://github.com/facebook/graphql/issues/334

https://graphql.org/learn/ covers Queries and Mutations but not Subscriptions.

Great task for someone who wants to contribute to the GraphQL community!

Start by reading the existing Queries and Mutations content in https://graphql.org/learn/ and review the linked GraphQL issue 334 for the intended Subscription material. Identify the Learn section files from those pages, then add a comparable Subscriptions section; done means the section is published and explains Subscriptions alongside the existing topics.

Written by the indexing model from the issue text.