[Examples] Update big table
# What does this PR do?

This PR updates the "big table of tasks" by

- adding a hyperlink to each of the example datasets
- add "image pretraining", and the Colab link for semantic segmentation
_The documentation is not available anymore as the PR was closed or merged._<|||||>Slightly related, wondering if we shouldn't link from this Big Table to the corresponding task in hf.co/tasks? (cc @merveenoyan)