update schema with later version

It seems that the Transkribus PAGE schema branch was modified several times over the years, and this repo only captures one moment. Could you please clarify?

In particular, what's the relation to [this version](https://raw.githubusercontent.com/Transkribus/TranskribusCore/c2f71e6c9844b17ddecc899513550b22aa511e0c/src/main/resources/xsd/pagecontent_extension.xsd)?