Fix and Clarify GraphQL API Getting Started instructions

What does this MR do?

Fixes errors and clarifies instructions for getting started with the GitLab GraphQL interface. Removes docs targeted at GraphQL API developers because it is redundent with the referenced style guide, and retains all relevant GraphQL API client developer docs.

Screenshots

NA

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

NA

Security

NA