.github: Add app-text/mandoc to automation

Author: krnowak

.github/workflows/portage-stable-packages-list

@@ -126,6 +126,7 @@ app-text/build-docbook-catalog
 app-text/docbook-xml-dtd
 app-text/docbook-xsl-ns-stylesheets
 app-text/docbook-xsl-stylesheets
+app-text/mandoc
 app-text/manpager
 app-text/sgml-common