[DOC] miscellaneous improvements

including:

- add a link to the tutorial in the main readme page
- remove learning page on 'env', move content on reference page
- add dynamic sub components section in component page
- add reference page on props
- split qweb page into engine/language pages
- move t-key information into qweb language page

doc/reference/environment.md

@@ -1,6 +1,14 @@
 # 🦉 Environment 🦉
 
-An environment is an object which contains a [`QWeb` instance](qweb.md). Whenever
+## Content
+
+- [Overview](#overview)
+- [Setting an Environment](#setting-an-environment)
+- [Content of an Environment](#content-of-an-environment)
+
+## Overview
+
+An environment is an object which contains a [`QWeb` instance](qweb_engine.md). Whenever
 a root component is created, it is assigned an environment (see
 [below](#setting-an-environment) for more info on this). This environment is
 then automatically given to each sub component (and accessible in the `this.env`
@@ -12,15 +20,20 @@ property).
    A    B
 ```
 
-This way, all components share the same `QWeb` instance.
+This way, all components share the same `QWeb` instance. Owl internally requires
+that the environment has a `qweb` key which maps to a
+[`QWeb`](qweb_engine.md) instance. This is the QWeb instance that will be used to
+render each templates in this specific component tree. Note that if no `QWeb`
+instance is provided, Owl will simply generate it on the fly.
 
-Note: some additional information about what should go into an environment
-can be found in the [learning section](../learning/environment.md).
+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.
 
 ## Setting an environment
 
 An Owl application needs an [environment](environment.md) to be executed. The
-environment has an important key: the [QWeb](qweb.md) instance, which will render
+environment has an important key: the [QWeb](qweb_engine.md) instance, which will render
 all templates.
 
 Whenever a root component `App` is mounted, Owl will setup a valid environment by
@@ -51,3 +64,42 @@ by simply doing this:
 ```js
 Component.env = myEnv; // will be the default env for all components
 ```
+
+## Content of an Environment
+
+Some good use cases for additional keys in the environment are:
+
+- 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() {
+  App.env = await myEnv();
+  const app = new App();
+  await app.mount(document.body);
+}
+```