Docs: Add 'import' section. (#18778)

* Docs: Add import section.

* Clean up

* Simplify import sample, remove dropdown and custom CSS.

* Clean up

* Clean up

docs/examples/en/loaders/GLTFLoader.html

@@ -25,6 +25,17 @@
 			and they require special handling during the disposal process. More information in the [link:https://threejs.org/docs/#manual/en/introduction/How-to-dispose-of-objects How to dispose of objects] guide.
 		</p>
 
+		<h2>Import</h2>
+
+		<p>
+			[name] is an add-on, and must be imported explicitly.
+			See [link:#manual/introduction/Installation Installation / Addons].
+		</p>
+
+		<code>
+			import { GLTFLoader } from 'three/addons/loaders/GLTFLoader.js';
+		</code>
+
 		<h2>Extensions</h2>
 
 		<p>