[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#4020

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

content/developer/reference/frontend.rst

@@ -15,7 +15,6 @@ JavaScript framework
     frontend/services
     frontend/hooks
     frontend/patching_code
-    frontend/javascript_cheatsheet
     frontend/javascript_reference
     frontend/mobile
     frontend/qweb