[FrameworkBundle] Simpler Kernel setup with `MicroKernelTrait`

[fabpot]

Q	A
Feature PR	symfony/symfony#57408
PR author(s)	@yceruto
Merged in	7.2

We created this issue to not forget to document this new feature. We would really appreciate if you can help us with this task. If you are not sure how to do it, please ask us and we will help you.

To fix this issue, please create a PR against the 7.2 branch in the symfony-docs repository.

Thank you! 😃

[javiereguiluz]

Closing as fixed in #19977.