fix: Change translation domain name for bundle translation and add an example

[jbonnier]

The documentation was misleading regarding the bundle translations.

While the page Symfony Bundles Documentation mentions that translation file names should use PascalCase — as shown in the example AcmeBlogBundle.en.xlf — the Best Practices for Bundles suggest that the translation domain should be acme_blog.

This inconsistency led me to believe that there might be some mechanism that automatically converts the snake_case domain into PascalCase, but that isn’t the case.

I reviewed several bundles by core Symfony developers (such as EasyAdmin and bundles from SymfonyCasts), and they do indeed appear to use PascalCase translation domain names.

[javiereguiluz]

Thank you @jbonnier.