Add `<?xml-model ?>` PI to custom RNC schema for DocBook files

This underrated processing-instruction (PI):

* associates the XML document with a schema
* is preferable over things like `<?oxygen RNGschema="…"?>`, because not everyone uses Oxygen  (I can only wish! ☺)
* is described in [Associating Schemas with XML documents 1.0 (Second Edition)](http://www.w3.org/TR/xml-model/)
* is more developer-friendly and self-explanatory than the `schemas.xml` files scattered throughout

Signed-off-by: Zearin <zearin@gonk.net>
Keep namespace declarations happy! ☺

Signed-off-by: Zearin <zearin@gonk.net>
Added a comment to `xproc.rnc` to help others validate easier

Signed-off-by: Zearin <zearin@gonk.net>
Squashed commit of the following:

commit 80b266efa4551cb33a0cbf2db06db5aba2e9f782
Author: Zearin <zearin@gonk.net>
Date:   Wed Sep 19 10:20:58 2012 -0400

    Use straight-quotes inside of `code` elements.

    Signed-off-by: Zearin <zearin@gonk.net>

commit 7690c0d74a3d1a56d731a2519041f77e286c3a85
Author: Zearin <zearin@gonk.net>
Date:   Tue Feb 21 10:12:33 2012 -0500

    Changed curly-quotes in example code to straight quotes.

    Silly code—curly quotes are for body text! ☺

commit 109005754df99b760fa9a88f34765bb29f1872b2
Author: Zearin <zearin@gonk.net>
Date:   Tue Feb 21 09:47:45 2012 -0500

    Converted to typographer’s quotes in body text.

Signed-off-by: Zearin <zearin@gonk.net>
Merge pull request #1 from Zearin/develop

Fixed quotes (body text: curly quotes, code samples: straight quotes)

Sorry it took so long. Been busy. :-/