Refactor GraphQL index page

doc/api/graphql/getting_started.md

doc/api/graphql/index.md

doc/api/graphql/reference/index.md

doc/ci/resource_groups/index.md

doc/ci/testing/code_quality.md

doc/development/api_graphql_styleguide.md

doc/development/fe_guide/index.md

doc/development/fe_guide/migrating_from_vuex.md

doc/security/user_file_uploads.md

tooling/graphql/docs/templates/default.md.haml