[Doctest] Setup, quicktour and task_summary
# What does this PR do?

This PR starts the work of re-enabling the doc tests and makes sure our documentation uses the latest version of the APIs. For the doctest setup, it registers the options we will need for `doctest` in the setup.cfg.

For the quicktour and task_summary, it tweaks all results to match the output of the code so the doctests pass for those two files, and removes the use of deprecated APIs (AutoModelWithLMHead) as well as favoring the call method of the tokenizers over the encode method.