Consider shortening parent page URLs in developer handbook

[kadamwhite]

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/