Minor: `@subtype` without `@type`

[skurzinz]

As discovered by @demigrigo, output TEI features zone[@subtype][not(@type)], which collides with the schematron assertions of https://www.tei-c.org/release/doc/tei-p5-doc/en/html/ref-att.typed.html.

<sch:rule context="tei:*[@subtype]">
<sch:assert test="@type">The <sch:name/> element should not be categorized in detail with @subtype unless also categorized in general with @type</sch:assert>
</sch:rule>

Unless there is some specific reasoning behind page2tei-0.xsl#L522 and #L527, this could be changed to simply @type?