feat: abstract messaging

[brian-baugher]

This change reduces what we export to solely what is needed. Stops exporting TARGET_ORIGINS and TemplateMetadata. Stops exporting useMessage hooks. Adds a new hook that uses a useMessage hook but makes it really easy to just grab the document.
Also updates docs to reflect this and Editor props. New usage:

const Edit: () => JSX.Element = () => {
  const entityDocument = useDocumentProvider();

  return (
    <DocumentProvider value={entityDocument}>
      <Editor document={entityDocument} puckConfigs={puckConfigs} />
    </DocumentProvider>
  );
};