Change the link to the Ascidoc manual to point to the quick ref instead of the main documentation page

Author: OlivierJourdan

en/modules/ROOT/pages/contributing.adoc

@@ -27,7 +27,7 @@ IMPORTANT: To edit content you need to have a https://github.com/[GitHub] accoun
 ====
 The content is written using the https://asciidoc.org/[AsciiDoc] markup language.
 
-To learn the basics and advanced features of this language, visit the https://docs.asciidoctor.org/asciidoc/latest/[AsciiDoc documentation website].
+To learn the basics and advanced features of this language, visit the https://docs.asciidoctor.org/asciidoc/latest/syntax-quick-reference/[AsciiDoc documentation website].
 ====
 
 == Writing conventions

en/modules/ROOT/pages/new-course.adoc

@@ -133,5 +133,5 @@ NOTE: the zip file and the PDF are set to be downloaded from the overview page.
 
 == Useful resources
 
-* https://docs.asciidoctor.org/asciidoc/latest/[Asciidoc syntax manual]
+* https://docs.asciidoctor.org/asciidoc/latest/syntax-quick-reference/[Asciidoc syntax manual]
 * https://docs.antora.org/antora/latest/[Antora documentation]