doc: restructure url.md

PR-URL: #12710
Reviewed-By: Joyee Cheung <[email protected]>
Reviewed-By: Daijiro Wachi <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Gibson Fahnestock <[email protected]>
Reviewed-By: Sam Roberts <[email protected]>

Author: jasnell