[IMP] developer/*: update js code to ES6

This PR updates javascript code to be up to date with the codebase standards:
- use the `static` keyword when possible
- proper owl imports
- removed unecessary `@odoo-module` directives

Task ID: 2258359

closes odoo/documentation#11466

Signed-off-by: Bastien Fafchamps (bafa) <bafa@odoo.com>

content/developer/reference/frontend/framework_overview.rst

@@ -152,9 +152,10 @@ client can use it. For example, the field registry contains all field components
 
 .. code-block:: javascript
 
+    import { Component } from "@odoo/owl";
     import { registry } from "./core/registry";
 
-    class MyFieldChar extends owl.Component {
+    class MyFieldChar extends Component {
         // some code
     }