Add example of how to get suggestions via AJAX

[romaricpascal]

Prompted by the review of #611, adds an example illustrating how to implement fetching suggestions from a remote server. It deals with the different bits of logistics around sending GET requests repeatedly:

• providing user feedback that results are being fetched, including after results are shown
• providing user feedback if the request errors
• debouncing the requests while users type to avoid overloading servers and waste bandwidth with requests whose results will get replaced after the next keystroke
• cancelling in-flight requests to ensure that only the results from the last request sent will show in the result, avoiding a response from an earlier request, but delayed by network and server timings, to replace that of a later request.

https://github.com/alphagov/accessible-autocomplete/assets/396367/8ad030ac-5b5b-4fc6-af9f-64b687d436d1

[tomayac]

This PR is gold and should be merged. Not showing a dynamic fetch() is my pet peeve with any autocomplete component.