Update web site and JS tutorials.

This updates the tutorial markdown, fixes up the literate programming
Python script, and updates the web site itself.

The doc build script now uses a Pipfile instead of "requirements.txt",
which I find less frustrating since it does not interfere with other
Python projects on your machine.

Fixes #2483.

web/docs/tutorial_triangle.md

@@ -203,8 +203,12 @@ this.camera = engine.createCamera();
 this.view = engine.createView();
 this.view.setCamera(this.camera);
 this.view.setScene(this.scene);
-this.view.setClearColor([0.1, 0.2, 0.3, 1.0]); // blue-green background
-this.resize(); // adjust the initial viewport
+
+// Set up a blue-green background:
+this.renderer.setClearOptions({clearColor: [0.0, 0.1, 0.2, 1.0], clear: true});
+
+// Adjust the initial viewport:
+this.resize();
 ```
 
 At this point, we're done creating all Filament entities, and the code should run without errors.