Create discussions-graphql-api.md (#17897)

* Create discussions-graphql-api.md

Here's the PR for our manual version of the Discussions GraphQL API documentation. Per the conversation in github/docs-team#318, the intention is to publish this guide as reference documentation until we have automation in place to generate these docs.

* style guide alignment

* rename article

* OAuth user -> OAuth app

* remove incorrect discussion description

* Use discussions product name

Co-authored-by: skedwards88 <skedwards88@github.com>

Author: evi-liu

content/graphql/guides/index.md

@@ -20,3 +20,5 @@ versions:
 {% link_in_list /using-the-explorer %}
 
 {% link_in_list /managing-enterprise-accounts %}
+
+{% link_in_list /using-the-graphql-api-for-discussions %}