[Update README] Add SegFormer and ViLT links
# What does this PR do?

As we now also have inference widgets for semantic segmentation & visual question answering, it makes sense to add them to the main README.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Very cool! We've got a finetune guide for semantic segmentation in the works at #18640 right now, with hopefully VQA to come soon.