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.

docs/webgl/tutorial_triangle.js

@@ -44,8 +44,13 @@ class App {
     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();
     this.render = this.render.bind(this);
     this.resize = this.resize.bind(this);
     window.addEventListener('resize', this.resize);