Add content to GraphQL documentation
What does this MR do?
This MR contains additions to the GraphQL API Style Guide for developers doing work on the GraphQL API and frontend engineers consuming it, within a GitLab context.
This is a complementary MR to !19010 (merged), which covers a 'getting started' GraphQL guide for GitLab GraphQL API consumers.
Edited by 🤖 GitLab Bot 🤖