[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/learning/environment.md

@@ -1,47 +0,0 @@
-# 🦉 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() {
-  App.env = await myEnv();
-  const app = new App();
-  await app.mount(document.body);
-}
-```

doc/learning/tutorial_todoapp.md

@@ -181,14 +181,14 @@ class App extends Component {
 }
 ```
 
-The template contains a [`t-foreach`](../reference/qweb.md#loops) loop to iterate
+The template contains a [`t-foreach`](../reference/qweb_templating_language.md#loops) loop to iterate
 through the tasks. It can find the `tasks` list from the component, since the
 component is the rendering context. Note that we use the `id` of each task as a
 `t-key`, which is very common. There are two css classes: `task-list` and `task`,
 that we will use in the next section.
 
 Finally, notice the use of the `t-att-checked` attribute:
-prefixing an attribute by [`t-att`](../reference/qweb.md#dynamic-attributes) makes
+prefixing an attribute by [`t-att`](../reference/qweb_templating_language.md#dynamic-attributes) makes
 it dynamic. Owl will evaluate the expression and set it as the value of the
 attribute.