Create a new Learn section for Subscriptions

[carolstran]

Description

Currently, the Learn section of graphql.org doesn't include any information about subscriptions. Right now, it's only in the specification, and that includes details for how to write and execute subscriptions.

It'd be great if we could get a section going and add some walk-throughs and tutorials so people can learn how to work with subscriptions 🎉

[j0k3r]

Duplicate of #584

[SURAJITSHAW]

Hello I really like to work on this. But I have a question after adding a subsection 'subscription' to learn section, I just add the contents that are here https://spec.graphql.org/draft/#sec-Subscription to the 'subscription'. Is it Okay ?

[hwillson]

As @j0k3r mentioned in #993 (comment), this is a duplicate of #584. Let's continue the discussion in that issue. Thanks!