Update guide

Author: stsewd

docs/guides/canonical.rst

@@ -31,6 +31,16 @@ Check your :guilabel:`Admin` > :guilabel:`Domains` page for the domains that we
 Implementation
 --------------
 
+If you are using :doc:`Sphinx </intro/getting-started-with-sphinx>`,
+Read the Docs will set the value of the html_baseurl_ setting (if isn't already set) to your canonical domain.
+
+.. _html_baseurl: https://www.sphinx-doc.org/page/usage/configuration.html#confval-html_baseurl
+
+If you are using :doc:`MkDocs </intro/getting-started-with-mkdocs>`,
+you can use the site_url_ setting.
+
+.. _site_url: https://www.mkdocs.org/user-guide/configuration/#site_url
+
 If you look at the source code for documentation built after you set your canonical URL,
 you should see a bit of HTML like this: