Add notes about how to use GraphQL Subscriptions in the Managed Service

[jamesbayly]

Add example code on how to use GraphQL Subscriptions in our Managed Service

Acceptance Criteria

• The example code should be added to the Subscriptions page on our docs
• You should suggest 2 or 3 example client libraries that customers can use
• The example code should be for the most common library, and should be written in Typescript