doc: inline the final_virtual_ptr example instead of linking to it

The explicit-`Registry` overload pointed at the default-registry overload's
example with a hardcoded page name, `final_virtual_ptr-08.adoc`. MrDocs
disambiguates overload pages with a content-derived hash, so adding the `any`
overloads renamed that page to `final_virtual_ptr-08ea.adoc` and the link went
dead.

It went dead silently: mrdocs-addons rewrites `xref:reference:` into a plain
`link:` on nested pages, to work around cppalliance/mrdocs#1245, and Antora
does not validate a link macro. Any cross-reference to an *overload* page is
therefore a link that rots without warning -- the two in macros.hpp are safe
only because macro page names carry no hash.

Pull in the snippet instead, with the same `include:` directive the
default-registry overload already uses. There is no page name left to rot, and
the example now comes from a file the build compiles and runs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: jll63

include/boost/openmethod/core.hpp

@@ -624,8 +624,7 @@ inline vptr_type null_vptr = nullptr;
 //!
 //! @par Example
 //!
-//! See [the default-registry overload](xref:reference:boost/openmethod/final_virtual_ptr-08.adoc#_example)
-//! for an example.
+//! include:virtual_ptr.cpp#non_polymorphic_classes;final_virtual_ptr
 //!
 //! @tparam Registry A @ref registry.
 //! @tparam Arg The type of the argument.