[IMP] developer: JavaScript tutorial: enhance chapter 1

This commit adds several small information on chapter 1 of the training
 owl:

- 1.1 How to get to the playground controller
- 1.2 Add a reference on sub component
- 1.4 How to activate the dev mode in owl

closes odoo/documentation#3177

Signed-off-by: Dardenne Florent (dafl) <dafl@odoo.com>

content/applications/general/developer_mode.rst

@@ -44,6 +44,8 @@ activate the debug mode.
    :align: center
    :alt: Command palette with debug command
 
+.. _developer-mode/url:
+
 Activate through the URL
 ========================
 

content/developer/howtos/discover_js_framework/01_components.rst

@@ -4,8 +4,7 @@ Chapter 1: Components
 
 This chapter introduces the `Owl framework <https://github.com/odoo/owl>`_, a tailor-made component
 system for Odoo. The main building blocks of OWL are `components
-<{OWL_PATH}/doc/reference/component.md>`_ and `templates
+<{OWL_PATH}/doc/reference/component.md>`_ and `templates <{OWL_PATH}/doc/reference/templates.md>`_.
-<{OWL_PATH}/doc/reference/templates.md>`_.
 
 In Owl, every part of user interface is managed by a component: they hold the logic and define the
 templates that are used to render the user interface. In practice, a component is represented by a
@@ -69,7 +68,12 @@ intended to quickly understand and practice the basics of Owl.
 =======================
 
 As a first exercise, let us implement a counter in the `Playground` component located in
-:file:`owl_playground/static/src/`.
+:file:`owl_playground/static/src/`. To see the result, you can go to the `/owl_playground/playground`
+route with your browser.
+
+.. tip::
+   The Odoo JavaScript files downloaded by the browser are minified. For debugging purpose, it's
+   easier when the files are not minified. Switch to :ref:`debug mode with assets <developer-mode/url>` so that the files are not minified.
 
 .. exercise::
 
@@ -89,11 +93,14 @@ As a first exercise, let us implement a counter in the `Playground` component lo
       :align: center
       :alt: A counter component.
 
+.. seealso::
+   `Video: How to use the DevTools <https://www.youtube.com/watch?v=IUyQjwnrpzM>`_
+
 2. Extract counter in a component
 =================================
 
 For now we have the logic of a counter in the `Playground` component, let us see how to create a
-sub-component from it.
+`sub-component <{OWL_PATH}/doc/reference/component.md#sub-components>`_ from it.
 
 .. exercise::
 
@@ -146,14 +153,20 @@ todos. This will be done incrementally in multiple exercises that will introduce
 
 The `Todo` component has an implicit API. It expects to receive in its props the description of a
 todo object in a specified format: `id`, `description` and `done`. Let us make that API more
-explicit. We can add a props definition that will let Owl perform a validation step in :ref:`dev
+explicit. We can add a props definition that will let Owl perform a validation step in `dev mode
-mode <developer-mode>`. It is a good practice to do that for every component.
+<{OWL_PATH}/doc/reference/app.md#dev-mode>`_. You can activate the dev mode in the `App
+configuration <{OWL_PATH}/doc/reference/app.md#configuration>`_
+
+ It is a good practice to do props validation for every component.
 
 .. exercise::
 
    #. Add `props validation <{OWL_PATH}/doc/reference/props.md#props-validation>`_ to the `Todo`
       component.
-   #. Make sure it passes in :ref:`dev mode <developer-mode>`.
+   #. Make sure it passes in dev mode which is activated by default in `owl_playground`. The dev
+      mode can be activated and deactivated by modifying the `dev` attribute in the in the `config`
+      parameter of the `mount <{OWL_PATH}/doc/reference/app.md#mount-helper>`_ function in
+      :file:`owl_playground/static/src/main.js`.
    #. Remove `done` from the props and reload the page. The validation should fail.
 
 5. A list of todos
@@ -165,7 +178,7 @@ list.
 .. exercise::
 
    #. Change the code to display a list of todos instead of just one, and use `t-foreach
-      <{OWL_PATH}/doc/reference/templates.md#loops>`_ in the template
+      <{OWL_PATH}/doc/reference/templates.md#loops>`_ in the template.
    #. Think about how it should be keyed with the `t-key` directive.
 
    .. image:: 01_components/todo_list.png
@@ -253,6 +266,21 @@ The final touch is to let the user delete a todo.
 .. exercise::
 
    #. Add a new callback prop `removeTodo`.
+
+   .. tip::
+
+      If you're using an array to store your todo list, you can use the JavaScript `splice` function
+      to remove a todo from it.
+
+      .. code-block::
+
+         // find the index of the element to delete
+         const index = list.findIndex((elem) => elem.id === elemId);
+         if (index >= 0) {
+               // remove the element at index from list
+               list.splice(index, 1);
+         }
+
    #. Insert :code:`<span class="fa fa-remove">` in the template of the `Todo` component.
    #. Whenever the user clicks on it, it should call the `removeTodo` method.