[REF] github page: move page to docs folder on master branch

Currently, some of the things on the playground must be changed directly
on the master branch while other things require checking out the
gh-pages branch and making the modifications there. Even when changes
need to be made on the master branch, all changes that are not direcly
in owl itself need to be copied by hand to the gh-pages branch.

This commit moves all files that exist on the gh-pages branch to the
master branch in the docs folder, this change will require configuring
the github page to use the docs folder instead of a separate branch, but
will make maintenance of the github page and playground easier going
forward.

docs/playground/samples/slots/slots.js

@@ -0,0 +1,49 @@
+// We show here how slots can be used to create generic components.
+// In this example, the Card component is basically only a container. It is not
+// aware of its content. It just knows where it should be (with t-slot).
+// The parent component define the content with t-set-slot.
+//
+// Note that the t-on-click event, defined in the Root template, is executed in
+// the context of the Root component, even though it is inside the Card component
+import { Component, useState, mount } from "@odoo/owl";
+
+class Card extends Component {
+  static template = "Card";
+  
+  setup() {
+    this.state = useState({ showContent: true });
+  }
+
+  toggleDisplay() {
+    this.state.showContent = !this.state.showContent;
+  }
+}
+
+class Counter extends Component {
+  static template = "Counter";
+  
+  setup() {
+    this.state = useState({val: 1});
+  }
+
+  inc() {
+    this.state.val++;
+  }
+}
+
+// Main root component
+class Root extends Component {
+  static template = "Root"
+  static components = { Card, Counter };
+  
+  setup() {
+    this.state = useState({a: 1, b: 3});
+  }
+
+  inc(key, delta) {
+    this.state[key] += delta;
+  }
+}
+
+// Application setup
+mount(Root, document.body, { templates: TEMPLATES, dev: true});