Document the exclusion of `src/DataFixtures/` for prod (dev dependency)

[GromNaN]

Fixture classes must implement the Doctrine\Common\DataFixtures\FixtureInterface. But this package is a "dev-dependency" that can be skipped for "prod" deployments.

When Symfony is configured to discover the src/ directory, the fixture classes are automatically registered as service in the DIC. But when the doctrine/data-fixtures package is not installed, the files in src/DataFixtures cannot be included by PHP. And the container compilation fails.

A solution is to scan this directory only in when@dev.

services:
    _defaults:
        autowire: true
        autoconfigure: true

    App\:
        resource: '../src/'
        exclude: '../src/DataFixtures/'

when@dev:
    services:
        App\DataFixtures\:
            resource: '../src/DataFixtures/'

Using #[WhenNot(env: 'prod')] is not possible since the class cannot be loaded.