Renders the full link page as HTML, without saving anything. Renders the posted body and
css; whatever is omitted comes from the saved draft (or the starter template if the saved
draft is empty or the project has no page template). The page is rendered for a sample link,
unless you pass link_id. The preview never redirects and never collects a fingerprint.

Asset URLs in the HTML are absolute, so it can be shown in a webview or a srcdoc iframe.
The HTML includes our page scripts, so show it in a sandboxed iframe
(<iframe sandbox="allow-scripts" srcdoc="...">) to keep them out of your own origin.

The request body is optional. When you send one, send Content-Type: application/json.
Saving, publishing and previewing share a limit of 60 requests per minute per API key.

Requirements: a Pro plan, page templates enabled for the project, and an API key of a project
owner or manager with the page_templates.show scope. Previewing with link_id also needs the
links.show scope.