Re-enable doctests for the quicktour
# What does this PR do?

This PR re-enables the doctests for the quicktour. It shouldn't be merged without its [companion PR](https://github.com/huggingface/doc-builder/pull/112) on the doc-builder side to properly recognize the new framework split comment and to remove the doctest flags on some lines.

I've adapted the example applying the pipeline on the whole superb dataset because:
1. it was way too long
2. writing a custom generator using an object not documented and not even in the main init of Transformers is too advanced for the quicktour.

I've also update the guide for doctests.
_The documentation is not available anymore as the PR was closed or merged._