Represent the HTML doctype of an HTML-5 page.
Example
>>> d = Doctype()
>>> d.serialize()
>>> '<!DOCTYPE html>'
All HTML documents must start with a <!DOCTYPE> declaration. The declaration is not an HTML tag. It is an "information" to the browser about what document type to expect.
Contrariwise to previous versions, HTML5 does not require any other information. Then this class does not accept any attribute or value.