fix(button): fixing the example in the notes tab, improving knobs - FRONT-893

[planctus]

PR description

This is better visible with the fix applied on the storybook plugin for notes in #350.
we recently worked on the button introducing knobs, but with the fix applied to the notes tab it was showing the icon object always, because we were setting it by default, with a "null" name which was enough not to get any markup generated.
Now the icon is only "added" when the select for the name is not "null", so when an icon is supposed to be shown.
I added knobs for the icon, with a new approach:
sometimes we want to show a knob without really letting the user interact with it, i was looking for a solution to "disable" a knob, but it's actually easier to just define it as a select where the only available value is the selected one. This way we show the property but we don't let the user change it, this has been applied to the icon size and the icon type.

QA Checklist

In order to ensure a safe and quick review, please check that your PR follow those guidelines:

• I have put the vanilla component as devDependencies
• I have put the specs package as devDependencies
• I have added the components directly used in the twig file (with include or embed) as dependencies
• My component is listed in @ecl-twig/ec-components's dependencies
• My variables naming follow the guidelines (snake case for twig)
• I have provided tests
• I have provided documentation (for the "notes" tab)
• If my local yarn.lock contains changes, I have committed it
• I have given my PR the proper label (pr: review needed to indicate that I'm done and now waiting for a review ,pr: wip to indicate that I'm actively working on it ...)