Add FFmpeg to about box in Qt Multimedia examples

"This software uses libraries from the FFmpeg project under the LGPLv2.1"

Add to the documentation of each Qt Multimedia example that may be using FFmpeg under the hood: "This example may use libraries from the FFmpeg project under the LGPLv2.1".

Rationale for not having this in the About box: Qt example are code examples, not product software. Showing this information in the documentation is therefore giving more visibility to developers using Qt Multimedia.