Add notes on setting env variables from server configs

[samdark]

No description provided.

[Copilot]

Pull Request Overview

This PR adds documentation for setting environment variables through web server configurations for Yii3 applications. It demonstrates how to configure the APP_ENV environment variable in three different web server setups.

• Adds environment variable configuration examples for nginx, nginx-unit, and Apache
• Includes explanatory notes about using environment variables with Yii3 application templates
• Provides production environment guidance for setting APP_ENV to prod

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 3 comments.

File	Description
cookbook/en/configuring-webservers/nginx.md	Adds fastcgi_param configuration and documentation for setting APP_ENV
cookbook/en/configuring-webservers/nginx-unit.md	Adds environment section to JSON config with APP_ENV setting
cookbook/en/configuring-webservers/apache.md	Adds SetEnv directive examples for both VirtualHost and .htaccess configurations

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