Replace `filterOutExhibitions`with `format=!exhibitions`

[rcantin-w]

(or something like it, but using ! as an exclusion marker)
This is how the Catalogue API excludes, so we should follow that model, it's much more flexible as well.
This is currently used in wc.org, so both would have to be supported until the change happens on the FE.
Then, delete filterOutExhibitions.

Note

Do we have to worry about external people using it?

Definition of done

• Change is done on the FE
• We don't support filterOutExhibitions anymore
• We can exclude any formats
• Documentation is updated (developers.wc.org)