update docs

Author: joaofreires

guide/src/format/configuration/renderers.md

@@ -158,6 +158,7 @@ The following configuration options are available:
 - **site-url:** The url where the book will be hosted. This is required to ensure
   navigation links and script/css imports in the 404 file work correctly, even when accessing
   urls in subdirectories. Defaults to `/`.
+- **use-site-url-as-root:** Prepend the `site_url` in links with absolute path.
 - **cname:** The DNS subdomain or apex domain at which your book will be hosted.
   This string will be written to a file named CNAME in the root of your site, as
   required by GitHub Pages (see [*Managing a custom domain for your GitHub Pages