[Sync EN] ext/operator: Add documentation (#4669)

Commit: `https://github.com/php/doc-en/commit/6043cd25418a3c45c64a4af118ee939c1835251b`
PR: `https://github.com/php/doc-en/pull/4669`

**Zu aktualisierende DE-Dateien**
- `language/operators.xml`

**Neue EN-Dateien (noch nicht übersetzt)**
- `appendices/extensions.xml`
- `reference/operator/book.xml`
- `reference/operator/overloading.xml`
- `reference/operator/setup.xml`
