[DevHub] Plugin Handbook AJAX doc does not mention or use the REST API

[tomjn]

Issue Description

The document that describes how to use AJAX in WordPress only refers to the old admin AJAX API, and not only doesn't describe AJAX with the REST API, but doesn't mention its existence. The REST API is a surprise for most users trying to debug AJAX, even though debugging with the REST API gives you human readable error messages where Admin AJAX prints 0

URL of the Page with the Issue

https://developer.wordpress.org/plugins/javascript/ajax/

Section of Page with the issue

Entire page

Why is this a problem?

While we shouldn't be encouraging the use of an older more difficult API, we should at the very least mention that a newer API exists

Suggested Fix

One of the following:

1. mention the REST API at the top and note that the document talks about admin-ajax.php which is an older API
2. rewrite the doc to work the same way but with a REST API endpoint via register_rest_route
3. add both REST API and Admin AJAX API examples

[github-actions]

Heads up - the "plugins" label was applied to this issue.

[estelaris]

There is a section dedicated to the REST API https://developer.wordpress.org/plugins/rest-api/

[tomjn]

@estelaris that existed when I opened the ticket too, that's not what the purpose of this issue was, and the problem I raised is still there. The AJAX page does not mention the REST API, link to the REST API section, or even suggest that it's possible to use it.

This erroneously gives the impression only admin-ajax.php is to be used for AJAX, which I have seen many people mistakenly believe.