[RFR] Added "How to Organize Configuration Files" cookbook

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | #3680

This is just the first draft of a new cookbook. Please, tell me anything that I should add, change or remove. Thanks!
@javiereguiluz, I did a PR about Directory Loading, if you want to take a look: https://github.com/symfony/symfony/pull/11059