[`Pix2Struct`] Add conditional generation on docstring example
# What does this PR do?

As discussed in https://github.com/huggingface/transformers/pull/23391#issuecomment-1549555750 - this PR adds an example for users to run conditional generation using pix2struct. In fact, users shouldn't add special tokens when pre-pending the text - therefore it should be explicitly mentioned in the docs (done on the aformentioned PR) but also on the example snippets.

cc @amyeroberts 

_The documentation is not available anymore as the PR was closed or merged._