Update config documentation

[Tigrov]

No description provided.

[Copilot]

Pull request overview

This PR updates the Yii3 definitive guide’s configuration documentation to reflect the current yiisoft/config workflow and the default application template’s configuration structure and examples.

Changes:

• Rewrites the “Config plugin” section to describe the merge plan/config loader flow and config/configuration.php.
• Updates the documented config/ directory layout and aligns examples (DI definitions, routes, events, params) with the template structure.
• Revises the “Package configs” section to describe vendor/root/environment layers and mentions yii-config-copy / yii-config-rebuild.

Comments suppressed due to low confidence (1)

src/guide/concept/configuration.md:47

• This block is also fenced as json but is only a fragment of composer.json (missing the outer { ... }). Consider wrapping it in a complete JSON object or clarifying it’s an excerpt (and using an appropriate code fence) so tooling/readers don’t treat it as invalid JSON.

```json
"extra": {
    "config-plugin-file": "config/configuration.php"
}

</details>