[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.xml

@@ -0,0 +1,33 @@
+<templates>
+  <div t-name="Card" class="card" t-att-class="state.showContent ? 'full' : 'small'">
+    <div class="card-title">
+      <t t-esc="props.title"/><button t-on-click="toggleDisplay">Toggle</button>
+    </div>
+    <t t-if="state.showContent">
+      <div class="card-content" >
+        <t t-slot="content"/>
+      </div>
+      <div class="card-footer">
+        <t t-slot="footer"/>
+      </div>
+    </t>
+  </div>
+
+  <div t-name="Counter">
+    <t t-esc="state.val"/><button t-on-click="inc">Inc</button>
+  </div>
+
+  <div t-name="Root" class="main">
+    <Card title="'Title card A'">
+      <t t-set-slot="content">Content of card 1...  [<t t-esc="state.a"/>]</t>
+      <t t-set-slot="footer"><button t-on-click="() => this.inc('a', 1)">Increment A</button></t>
+    </Card>
+    <Card title="'Title card B'">
+      <t t-set-slot="content">
+        <div>Card 2... [<t t-esc="state.b"/>]</div>
+        <Counter />
+      </t>
+      <t t-set-slot="footer"><button t-on-click="() => this.inc('b', -1)">Decrement B</button></t>
+    </Card>
+  </div>
+</templates>