Consider shortening parent page URLs in developer handbook

It was suggested in slack that we shorten the URLs for the intermediate pages in the REST API handbook, so e.g.

> https://developer.wordpress.org/rest-api/extending-the-rest-api/
>
> becomes 
>
> https://developer.wordpress.org/rest-api/extending/

and

> https://developer.wordpress.org/rest-api/using-the-rest-api/
>
> becomes
>
> https://developer.wordpress.org/rest-api/using/
