Documentation about plugin needs to be more precise

### Feature description
This page : https://docs.tabris.com/latest/custom-widgets.html needs to be more precise on how to call a tabris plugin installed on our application.
I wanted to import my tabris plugin into my app.js file but could not guess how to do it. Fortunately a friend of mine told me.

### Proposed solution
I had to do : const {MyPlugin} = require('tabris'); // how was I suppose to know that the plugin was inside "tabris" ?

also I'm not even sure that If I did in my app.js
var myplugin = new MyPlugin()
it would have worked... did'nt tried yet. But if it works you should explain it too in your document.

Thank you!