System CLI: download-vendor command feedback

[ssddanbrown]

As per our blogpost here: https://www.bookstackapp.com/blog/php-dependency-improvements/

This is a thread to collection feedback about use of the download-vendor System CLI action, available from BookStack v25.02.1.

Once we've had a few rounds of feedback and confirmation of stable use, our update & install guidance, including future scripts, can be updated to make use of this instead of composer.