Improve the GraphQL explorer experience

Background

Users starting to use the GitLab GraphQL API and/or GraphQL itself have little guidance on how to get start once they reach the GraphiQL explorer, this makes it hard for our users to understand how to use it.

Description

This is our current page upon landing:

https://gitlab.com/-/graphql-explorer

Examples of things we may improve:

• It has an commented out query so you’re left not knowing how to execute your first query
• There is no link the our GraphQL documentation
• There is no logo or GitLab look and feel