Symfony-docs: [FrameworkBundle] add cache.app.simple psr simple cache

[javiereguiluz]

See https://github.com/symfony/symfony/pull/25710.

@dmaicher asked in the original PR if he should update the docs. If you can't do this, we'd need at least some explanation about what this change does exactly for end users and, if needed, a quick example. Thank you!

[dmaicher]

I can look into it :blush:

Where should it go though? I cannot find anything in the docs for the cache.app service?

https://symfony.com/doc/current/components/cache.html

[nicolas-grekas]

This issue is deeper: we need a new page to document the framework bundle integration in the cache component actually.

[nicolas-grekas]

We're going to fade out PSR16 actually, so no need to add any doc for it.
See https://github.com/symfony/symfony/issues/28918