Move ADL interface to package.

Also, fix up its docs.  Phew, those had old and badly misleading links.

adl.go

@@ -1,7 +1,7 @@
 package ipld
 
 import (
-	"github.com/ipld/go-ipld-prime/datamodel"
+	"github.com/ipld/go-ipld-prime/adl"
 )
 
-type ADL = datamodel.ADL
+type ADL = adl.ADL

adl/interface.go

@@ -0,0 +1,25 @@
+package adl
+
+import (
+	"github.com/ipld/go-ipld-prime/datamodel"
+)
+
+// ADL is an interface denoting an Advanced Data Layout,
+// which is something that supports all the datamodel.Node operations,
+// but may be doing so using some custom internal logic.
+//
+// For more details, see the docs at
+// https://ipld.io/docs/advanced-data-layouts/ .
+//
+// This interface doesn't specify much new behavior, but it does include
+// the requirement of a way to tell an examiner about your "substrate",
+// since this concept does seem to be present in all ADLs.
+type ADL interface {
+	datamodel.Node
+
+	// Substrate returns the underlying Data Model node, which can be used
+	// to encode an ADL's raw layout.
+	//
+	// Note that the substrate of an ADL can contain other ADLs!
+	Substrate() datamodel.Node
+}

datamodel/node.go

@@ -165,19 +165,6 @@ type Node interface {
 	Prototype() NodePrototype
 }
 
-// ADL represents an Advanced Data Layout, a special kind of Node which
-// implements custom logic while still behaving like an IPLD node.
-//
-// For more details, see the docs at
-// https://github.com/ipld/specs/blob/master/schemas/authoring-guide.md.
-type ADL interface {
-	Node
-
-	// Substrate returns the underlying Data Model node, which can be used
-	// to encode an ADL's raw layout.
-	Substrate() Node
-}
-
 // NodePrototype describes a node implementation (all Node have a NodePrototype),
 // and a NodePrototype can always be used to get a NodeBuilder.
 //