Improve discoverability of starter documentation by adding a dedicate Boot Starter section in the reference

[gmuth]

spring.graphql options and it's default values need to be looked up in class org.springframework.boot.autoconfigure.graphql.GraphQlProperties.

It would be great to have the meaning and default values documented in the reference documentation.

[bclozel]

The properties class is part of Spring Boot, and it's documented automatically like all configuration properties. If your IDE supports it, you'll also get auto-completion when editing your application.yml file.

[rstoyanchev]

I've added a small "Boot Starter" section to our reference with a short summary. It includes links to all GraphQL related sections in the Boot reference, which are otherwise not listed anywhere in one place. This should make it easier to find the relevant information. It also gives us a local section to refer to whenever the Boot Starter is mentioned in our reference.