Put how to do templates in the use guide

[luiz]

Write a section explaning how one can implement page templates using an abstract class. Like this:
abstract class Template implements Page {
public NestedElement header() {
// return header code here
}
public NestedElement footer() {
// return footer code here
}
public abstract NestedElement content();
public final Tag render() {
return html().with(
header(),
content(),
footer());
}
}
class IndexPage extends Template {
public NestedElement content() {
// return content here
}
}