Looking at the docs you linked, it’s more focused to normal web-hosting where you don’t have access to change the config and what hostnames point where.
In Symbiosis it’s fairly simple, and can be done either by adding a symlink from the new subdomain to the existing domain where WordPress is set up, or if you want to have separate SSL certs and mail/etc configurations, you can symlink the htdocs/ directory.
Heres a couple of examples, using
example.com as the main site, and
subdomain.example.com as the new subdomain.
Create a symlink for the new name, pointing to the existing name.
$ ln -s /srv/example.com /srv/subdomain.example.com
This will alias both the web and other traffic, so mail sent to firstname.lastname@example.org will be the same as email@example.com (and any other subdomains), and the automatic SSL certificate will have subdomain.example.com added to it after a while.
Create the directory (move/remove the automatically created htdocs in Symbiosis Stretch) and link the new htdocs/ directory to the existing setup, so they share the same files.
$ mkdir -p /srv/subdomain.example.com/public
# The line below is needed in Symbiosis Stretch as the htdocs directory is automatically populated - if you're using Symbiosis Jessie, you can skip it.
$ if [ -d /srv/subdomain.example.com/public/htdocs ]; then mv /srv/subdomain.example.com/public/htdocs /srv/subdomain.example.com/public/htdocs_old; fi
$ ln -s /srv/example.com/public/htdocs /srv/subdomain.example.com/public/htdocs
This will set up what is in effect a separate configuration as far as Symbiosis is concerned, with it’s own SSL certificate, mail configuration and so on.
Hopefully the above all makes sense, but if you have any other questions just drop a reply here!