docs: Move boxed type documentation to Markdown

And add some additional new content and examples.

Helps: #3037

gobject/gboxed.c

@@ -33,35 +33,6 @@
 #include "gvaluearray.h"
 #include "gvaluecollector.h"
 
-
-/**
- * SECTION:gboxed
- * @short_description: A mechanism to wrap opaque C structures registered
- *     by the type system
- * @see_also: #GParamSpecBoxed, g_param_spec_boxed()
- * @title: Boxed Types
- *
- * #GBoxed is a generic wrapper mechanism for arbitrary C structures.
- *
- * The only thing the type system needs to know about the structures is how to
- * copy them (a #GBoxedCopyFunc) and how to free them (a #GBoxedFreeFunc);
- * beyond that, they are treated as opaque chunks of memory.
- *
- * Boxed types are useful for simple value-holder structures like rectangles or
- * points. They can also be used for wrapping structures defined in non-#GObject
- * based libraries. They allow arbitrary structures to be handled in a uniform
- * way, allowing uniform copying (or referencing) and freeing (or unreferencing)
- * of them, and uniform representation of the type of the contained structure.
- * In turn, this allows any type which can be boxed to be set as the data in a
- * #GValue, which allows for polymorphic handling of a much wider range of data
- * types, and hence usage of such types as #GObject property values.
- *
- * #GBoxed is designed so that reference counted types can be boxed. Use the
- * type’s ‘ref’ function as the #GBoxedCopyFunc, and its ‘unref’ function as the
- * #GBoxedFreeFunc. For example, for #GBytes, the #GBoxedCopyFunc is
- * g_bytes_ref(), and the #GBoxedFreeFunc is g_bytes_unref().
- */
-
 static inline void              /* keep this function in sync with gvalue.c */
 value_meminit (GValue *value,
 	       GType   value_type)