Add XSD to all monodoc XML as they are generated

[lobrien]

The root element of the monodoc XML should specify the monodoc-ecma.xsd:

<Type Name="Foo" FullName="Bar.Foo" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://github.com/mono/api-doc-tools/mdoc/Resources/monodoc/1.0/monodoc-ecma.xsd">...</Type>"

(I am not sure about best practice for the schema. I think it's something like the above: $"{base}/{schemaname}/{semantic MAJOR.MINOR}/{xsdfile}" but I may be wrong.)

[joelmartinez]

Github asks you to avoid linking directly to files for "production" use ... so if we were going to do this, we'd want to host these XSD files somewhere.