For 'Hello World' example we will need as little as just two tiny files:
.site.xml
and
.content/helloworld.html
The result:
Note, that there is some other content on this result page. This is because SiteXML:PHP engine, which was used for this example, has built-in theme with header, footer and navigation. Even if you do not supply any theme, the site has navigation - the only required site's attribute, without which site cannot exist.
Thus, by putting just two files you get a small, but functioning site, which has header, footer, navigation, which allows you to login using OpenId and edit site content right in you browser, depending on SiteXML client.
Demonstration: hello.sitexml.info