docs: replace graphiql playgrounds in GraphQL Reference with code blo…

…cks (#32543)

* chore(docs): replace GraphiQL iframes with code blocks

* chore(docs): update copy in GraphQL reference

* Update docs/docs/graphql-reference.md

Co-authored-by: Florian Kissling <21834+fk@users.noreply.github.com>

* Update docs/docs/graphql-reference.md

Co-authored-by: Florian Kissling <21834+fk@users.noreply.github.com>

* chore(docs): fix typo

* docs: update gatsbyjs.com link to use relative path

* docs: fix grammar, and make link text descriptive (for a11y)

* docs: add missing comma

* docs: clean up heading structure

Co-authored-by: Florian Kissling <21834+fk@users.noreply.github.com>
Co-authored-by: Megan Sullivan <megan@gatsbyjs.com>