[DOC] doc: update/add documentation on js framework

The state of the JS documentation was quite sad, not updated in a long
time.  With this commit, we rework a lot of the JS doc.  Most notably,
we add a section 'Javascript Cheatsheet' (help on some common
customization tasks) and rewrite the 'Javascript Reference' totally.

Note that the JS reference page also contains a reference on all field
widgets.

This work is just the beginning, it is probably not complete, but at
least we will have a foundation.

addons/web/static/src/js/fields/basic_fields.js

@@ -1757,7 +1757,7 @@ var StatInfo = AbstractField.extend({
 
 var FieldPercentPie = AbstractField.extend({
     template: 'FieldPercentPie',
-    supportedFieldTypes: ['integer'],
+    supportedFieldTypes: ['integer', 'float'],
 
     /**
      * Register some useful references for later use throughout the widget.