[IMP] create JavaScript howtos

The JavaScript cheatsheet is outdated, we therefore remove it and
replace it by multiple howtos:

- Create a view from scratch
- Extending an existing view
- Create a field from scratch
- Extend an existing field
- Create a client action

There is other subjects to introduce as the web framework is big. Other
future contributions will cover them.

closes odoo/documentation#4018

X-original-commit: 7e4435deb8
Signed-off-by: Dardenne Florent (dafl) <dafl@odoo.com>

content/developer/howtos.rst

@@ -9,6 +9,9 @@ How-to guides
     :titlesonly:
 
     howtos/scss_tips
+    howtos/javascript_field
+    howtos/javascript_view
+    howtos/javascript_client_action
     howtos/web_services
     howtos/company
     howtos/accounting_localization
@@ -23,6 +26,21 @@ How-to guides
 
       Follow this guide to keep the technical debt of your CSS code under control.
 
+   .. card:: Customize a field
+      :target: howtos/javascript_field
+
+      Learn how to customize field components in the Odoo JavaScript web framework.
+
+   .. card:: Customize a view type
+      :target: howtos/javascript_view
+
+      Learn how to customize view types in the Odoo JavaScript web framework.
+
+   .. card:: Create a client action
+      :target: howtos/javascript_client_action
+
+      Learn how to create client actions in the Odoo JavaScript web framework.
+
    .. card:: Web services
       :target: howtos/web_services