Use &url-docs-gh;

docbook/gnc-docbookx.dtd

@@ -45,8 +45,9 @@ https://www.w3.org/2003/entities/2007/w3centities-f.ent
 <!-- Fixme: a dirty hack to reference the other document
             Replace them after olink setup               -->
 <!ENTITY url-docs-draft "https://code.gnucash.org/docs/">  <!-- used while developing and for the API-docs -->
+<!ENTITY url-docs-gh "https://gnucash.github.io/gnucash-docs/">  <!-- another nightly build of the docs -->
 <!ENTITY url-docs-release "https://www.gnucash.org/docs/v&series-unstable;/">  <!-- used on release -->
-<!ENTITY url-docs "&url-docs-release;">  <!-- One of the last. ToDo: Should this become a CMake option? -->
+<!ENTITY url-docs "&url-docs-gh;">  <!-- One of the last. ToDo: Should this become a CMake option? -->
 
 <!--
   1. b) Publication - Translators can add transliterations