Add Document Image Transformer (DiT)
# What does this PR do?

This PR adds the conversion script used to convert DiT checkpoints from the [original repo](https://github.com/microsoft/unilm/tree/master/dit) to the hub.

It also adds a dedicated docs page (referring to the docs of BEiT).
_The documentation is not available anymore as the PR was closed or merged._<|||||>Failing test is unrelated, therefore merging.