[Book][ServiceContainer] Added docs for YAML '@@' escaping

This PR deprecates https://github.com/symfony/symfony-docs/pull/2286

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes https://github.com/symfony/symfony/pull/7357
| Applies to    | 2.1
| Fixed tickets | https://github.com/symfony/symfony/issues/4889