Merge branch 'bw-graphql-guide' into 'master'

Create a GraphQL Development Guide

See merge request gitlab-org/gitlab!38953

doc/development/graphql_guide/index.md

+# GraphQL development guidelines
+
+This guide contains all the information to successfully contribute to GitLab's
+GraphQL API. This is a living document, and we welcome contributions,
+feedback, and suggestions.
+
+## Resources
+
+- [GraphQL API development style guide](../api_graphql_styleguide.md): development style guide for
+  GraphQL.
+- [GraphQL API documentation style guide](../documentation/styleguide.md#graphql-api): documentation
+  style guide for GraphQL.
+- [GraphQL API](../../api/graphql/index.md): user documentation for the GitLab GraphQL API.