title: How to create a default `creationDate` for a document in sanity.io?
body: <p>I have checked the <a href="https://www.sanity.io/docs/schema-types/datetime-type" rel="nofollow noreferrer">documentation</a> of how to do this, but I have not found an answer.</p>
  
  <p>Basically, when a document is created/published, I want to set a <code>createdDate</code> for the document (as a read-only/hidden field).</p>