Added docling option to load files

[ParisNeo]

Docling is a superior tool to load data from files and format them.

I added a new option to select document loading engine.

I did put docling as the default one as my tests confirml its superiority compared to PyPDF.

I updated the documentation too.

I made the arguments visible to any class that needs them so that we can use them when needed.

[danielaskdd]

That's great. Do you check if Docling is blocking frontend any more?