[TwigBundle] Document twig.safe_class resource tag

[lacatoire]

Documents the new 8.1 twig.safe_class resource tag from symfony/symfony#63929: bundles can now mark a class as safe for Twig's escaper (per strategy: html, js, css, etc.) without decorating the environment configurator. Adds the entry to reference/dic_tags.rst, an anchor on the existing "Tagging Non-Service Classes" section in service_container/tags.rst, and a cross-reference tip in the templates.rst output-escaping section.

Fixes #22307

[GromNaN]

Thanks for this contribution! 🙏

Unfortunately, this feature isn't available yet: it depends on twigphp/Twig#4795, which is still pending. This PR should be put on hold until the Twig PR is merged. Once it is, the documentation will also need to specify the minimum required Twig version.