Add long answer candidates to natural questions dataset
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_4368). All of your documentation changes will be reflected on that endpoint. Once we have added `long_answer_candidates` maybe it would be worth to also add the missing `candidate_index` (inside `long_answer`). What do you think, @seirasto ? Also note the "Data Fields" section in the README is missing the `long_answer` field.

Moreover, there is no instance example in "Data Instances" section. We could either make these fixes in this PR or in a subsequent PR. @albertvillanova I've added the missing fields and updated the README to include a data instance and some other things.  Great! I've made the updates to align the README. Please let me know if I missed anything. As there were many minor little fixes, I thought it would be easier to fix them directly. I think the loading script is OK now. If it is also validated by another datasets maintainer, I could run the generation of the pre-processed data and then merge this PR into master (once all the tests are green).

CC: @lhoestq 
This is a modification of the Natural Questions dataset to include missing information specifically related to long answer candidates. (See here: https://github.com/google-research-datasets/natural-questions#long-answer-candidates). This information is important to ensure consistent comparison with prior work. It does not disturb the rest of the format . @lhoestq @albertvillanova 