docs: document the extensionworkers api

[y-l-g]

Solve https://github.com/php/frankenphp/issues/1967 I propose this documentation for the ExtensionWorkers API. I have included it in the Extensions section but as the page is becoming quite large (over 1000 lines), would it be better to create a dedicated section for it?

[henderkes]

would it be better to create a dedicated section for it?

Yes. We shouldn't have pages that long in the docs.