Thank you all for your advice, but I can't manage to solve my problem. I'll let you know more about what I need and what I have done already:
I have a web application working on my Zentyal server. It can be accessed through the url mypage.mydomain.com/nameoftheapplication/public/.
Since I want to make it easier for the users of the application to access it, I needed to change the documentroot for mypage.mydomain.com to point to the public directory. To do so, I created the file /etc/apache2/sites-available/user-ebox-mypage.mydomain.com/documentroot with the right documentroot directive. Now, when I go to mypage.mydomain.com, the homepage of my application is displayed.
Now, what I need is to get the same behavior when I go to
www.mypage.mydomain.com (that is, access the same homepage). As of now, when I go to that last URL, I get the message
It works!
This is the default web page for this server.
The web server software is running but no content has been added, yet.
(I don't know if it's a default behavior of Zentyal because as far as I can tell, there is no www alias configured for this domain).
I think that what I am trying "the Zentyal way" would be accomplished like this in a non-zentyal web server:
<VirtualHost *>
ServerName mypage.mydomain.com
ServerAlias www.mypage.mydomain.com
DocumentRoot "/srv/www/mypage.mydomain.com/nameoftheapplication/public/"
</VirtualHost>
I have looked all around the Zentyal interface, and I guess there must be something at the DNS section to make the www URL point to the same folder than the non-www URL, but can't find it (answering innocenti_jr, I'm not very experienced at managing Apache, though up to now I've been able to google my way).
I expect I made my case clearer (instead of more messy, with so much info). I'm also glad to have users endorsed by Zentyal staff trying to help me.
Thanks again for your attention