Introducing Dynamic URLs

The idea behind this PR is to have always our documentation updated to the latest versions as we advance in features but we forget to update the links to the CDN.

Let's assume we are documenting a library that uses Auth0 Widget. Then we will have something like this:

>Include the following html tag:
>```html
><script src="//cdn.auth0.com/w2/auth0-widget-5.0.js"> </script>
>```

Then, once we update Auth0 docs to use Auth0 Widget to use `6.0` (changing auth0-configuration) and we forget to do the change our customers are still using the old version and not getting the new improvements. By accepting this change, we will have this:

>Include the following html tag:
>```html
><script src="@@widget_url_no_scheme@@"> </script>
>```

That will render into the following (by getting the required information from `auth0-configuration`):
>Include the following html tag:
>```html
><script src="//cdn.auth0.com/w2/auth0-widget-5.0.js"> </script>
>```

Thoughts?