Export to PDF by using LibreOffice's binary

[Pierstoval]

So far, a few PDF exporters have a bridge in PHPWord, but the output depending on the library can be a bit inconsistent (latest example to date: #2534, apparently).

With LibreOffice, we can use a script like the ones suggested in the official docs: https://help.libreoffice.org/latest/en-US/text/shared/guide/pdf_params.html

The documentation must be taken in account because the soffice binary might have several different versions, leading to a possible maintenance burden on the parameters depending on the versions.
(as example, on one of my project, I convert some OpenDocument files into PDF via this command: soffice --headless --convert-to pdf:writer_pdf_Export --outdir {{output}} {{input}} ).

Issue asked gently by @Progi1984 , so here it is 👍