[DOC] update the documentation to new config and env

part of #306

doc/learning/environment.md

@@ -0,0 +1,48 @@
+# 🦉 Environment 🦉
+
+An environment is an object which contains a [`QWeb` instance](../reference/qweb.md).
+Whenever a root component is created, it is assigned an environment (see the
+reference section on [environment](../reference/environment.md). This environment
+is then automatically given to each sub components (and accessible in the `this.env` property).
+
+The environment is mostly static. Each application is free to add anything to
+the environment, which is very useful, since this can be accessed by each sub
+component.
+
+Some good use cases for the environment is:
+
+- some configuration keys,
+- session information,
+- generic services (such as doing rpcs, or accessing local storage).
+
+Doing it this way means that components are easily testable: we can simply
+create a test environment with mock services.
+
+For example:
+
+
+```js
+async function myEnv() {
+    const templates = await loadTemplates();
+    const qweb = new QWeb({templates});
+    const session = getSession();
+
+    return {
+        _t: myTranslateFunction,
+        session: session,
+        qweb: qweb,
+        services: {
+            localStorage: localStorage,
+            rpc: rpc,
+        },
+        debug: false,
+        inMobileMode: true,
+    };
+}
+
+async function start() {
+    owl.config.env = await myEnv();
+    const app = new App();
+    await app.mount(document.body);
+}
+```