valadoc: Use HTML5 doctype in HTML doclet

Al Thomas requested to merge astavale/vala:valadoc-html5 into master

Patch removes the XML declaration in the HTML doclet and adds the HTML5 doctype: <!DOCTYPE html>

After many years of complex doctypes the consensus now is this simple one is all that is needed. This triggers standards compliance mode in all web browsers, instead of quirks mode. That is a good starting point for writing any CSS and may have an impact on strange browser rendering bugs like #644 (closed)

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