feat: describe how to set mail data via subscriber

[ROBJkE]

Extend 'Add Data to Mails' guide

[Isengo1989]

Thx for the PR @ROBJkE - please check the Copilot comments. As it mentions this can be misleading in terms of overriding the whole template.

Consider using smth like this instead and briefly explain the diff between the two, or just leave setTemplateData out. If you are not modifying existing data, this can be removed.

$templateData = $event->getTemplateData();
    
    // Add custom data
    $templateData['customValue'] = 'some data';
    
    // Set the modified template data
    $event->setTemplateData($templateData);

[ROBJkE]

Added inline comments, removed unnecessary constructor, removed setTemplateData() example as it could be misleading

[Copilot]

Pull Request Overview

This PR extends the "Add Data to Mails" guide by adding documentation for an alternative approach to add mail data using event subscribers instead of service decoration. The guide now provides two methods for customizing mail data in Shopware plugins.

• Reformatted existing code examples to use code groups for better presentation
• Added a new section documenting how to use event subscribers for mail data manipulation
• Updated code formatting and template syntax for improved readability

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[Isengo1989]

Thank you for the PR @ROBJkE 🥇