Force correct HTML doctype when mummifying pages.

When parsing XML files with other doctypes, e.g. legacy XHTML 1.1 files:

Guise will use the same doctype when producing "HTML":

This was never noticed until now because either source files had been using the HTML5 doctype, or a template was in place that used the HTML5 doctype. We instead want an (X)HTML5 doctype declaration:

This will probably require a new feature in the GlobalMentor XML/HTML serializer to allow specification of the doctype.