docs: drop XHTML 1.0 validation of website
    
    The HTML pages are currently validated against an XHTML 1.0 DTD.
    This makes it impossible to take advantage of features that are
    introduced in HTML 5, because they'll fail validation.
    
    There is intentionally no DTD defined for HTML 5, so there's no
    alternative to XHTML 1.0 DTD that we could switch to. The only
    options are to stick with XHTML 1.0 forever, or drop the DTD
    validation, and we pick the latter.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>