Explicit handling of external documentation references

In order to have different formatting of external documentation references in our generated GraphQL documentation and in other clients (such as GraphIQL), this MR treats documentation references as a separate first-class argument to fields.

This allows us to capture the information, and render it differently in different environments.

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.