feat(performance): update Souin setup & enable Cache-Control public in docs

[laurent-bientz]

Hello,

This pull request updates Souin setup in docs, see https://github.com/dunglas/frankenphp/issues/976#issuecomment-2296050761

[dunglas]

By the way, the config can now be simplified:

Before:

{
    order cache before rewrite
    ...
    cache {
        api {
            souin
        }
    }
}

After:

{
    cache
    # ...
}

# ...

[laurent-bientz]

@dunglas I followed your recommendations, tell me if everything is ok

[dunglas]

Thank you!!