[php-src] PR #23663: docs: write docs for FCI/FCC structs and PHP callables API.

Pull Request: https://github.com/php/php-src/pull/23663 Author: Girgias Partly based on my writing for the PHP Internals Book. I need to confirm a few things again in regards to the zend_object *object fields and params as I think the behaviour of it in the FCI and FCC structs *is* different. @arnaud-lb could you add an explanation for the new consumed_args field of the FCI? As I *vaguely* know what it does but not how one is meant to use it, and also proof read the rest of what I wrote. :) I'm also aware that the tables for the FCI section look like trash, as I still need to convert the inline code to block code, but it was frustrating enough to get it work in the first place (I'm not a fan of whitespace sensitive markup languages...) I might add a section specifically dealing with Closure and the get_closure hook/API as this comes up semi-often. @TimWolla you might be interested in reviewing this to see if this answers most of your questions regarding the FCI/FCC APIs. :)