doc(book): explain the schema importing mechanism

This section was still empty and despite even now being somewhat
incomplete, at least provides some basic explanation of the importing
system.

book/src/reference/schema/imports.md

@@ -1,5 +1,29 @@
 # Imports
 
-## Schema
+When splitting data structures into multiple Schema files, imports are used to reference the types defined within. Imports are declared with the `use` statement.
 
-<<< imports/basic.mabo
+These come in two flavors:
+
+- Import a specific type.
+
+  <<< imports/struct.mabo
+
+- Import only a module.
+
+  <<< imports/module.mabo
+
+Individual elements forming the import path are separated by a double-colon `::`. The first element is name of the external schema, and all intermediate elements are modules.
+
+The last element is either omitted, which results in bringing the whole module into scope, or a specific type in that module or root schema.
+
+Importing a module can help to reduce repetition if the module path is deep. Another use case is the avoidance of duplicate type names. For example:
+
+## Scoping example
+
+A simple example with a somewhat deeply nested module structure, which we'll use in the next step. Let's consider this file to be named `other.mabo`.
+
+<<< imports/other.mabo
+
+When importing the above schema, we bring the `addresses` module into scope and can reference all the contained types:
+
+<<< imports/scoping.mabo

book/src/reference/schema/imports/module.mabo

@@ -0,0 +1 @@
+use other::my_module;

book/src/reference/schema/imports/other.mabo

@@ -0,0 +1,8 @@
+mod users {
+    mod addresses {
+        struct Street {
+            name: string @1,
+            house_no: string @2,
+        }
+    }
+}

book/src/reference/schema/imports/scoping.mabo

@@ -0,0 +1,5 @@
+use other::users::addresses;
+
+struct MyStruct {
+    street: addresses::Street @1,
+}

book/src/reference/schema/imports/struct.mabo

@@ -0,0 +1 @@
+use other::my_module::MyStruct;