Doc: AppSync

[fwang]

• https://ion.sst.dev/docs/component/aws/app-sync/
• https://ion.sst.dev/docs/component/aws/app-sync-data-source/
• https://ion.sst.dev/docs/component/aws/app-sync-function/
• https://ion.sst.dev/docs/component/aws/app-sync-resolver/