html_export_header_link

Html export should accept an optional header link ( href + label ) rendered in the topbar ahead of section navigation.
Hosts such as the marketing site build can set this so readers can return to the parent site from any exported page.
When omitted, the topbar shows only the export section nav.
Site embed consumption: spec_html_export.
Often paired with html_export_url_base when the export is mounted under a parent static site.

Status: done

Interactive page: html_export_header_link