Add Wikimedia pre-processed datasets
As we are planning to stop using Apache Beam (our `datasets.BeamBasedBuilder`) for the generation of some datasets (including [Wikipedia](https://huggingface.co/datasets/wikipedia/blob/main/wikipedia.py)), I have been working on [wikimedia/wikipedia](https://huggingface.co/datasets/wikimedia/wikipedia) to:
- Port the Wikipedia generation script to use `datasets.GeneratorBasedBuilder` instead and place it under the "script" branch: https://huggingface.co/datasets/wikimedia/wikipedia/tree/script
- Improve the efficiency of the code and make it highly parellizable. See:
  - [Parallelize dataset generation over multistreams](https://huggingface.co/datasets/wikimedia/wikipedia/commit/610c55864586dbdad7ac5a13c21a367bb000a1d3)
  - [Parallelize data downloading](https://huggingface.co/datasets/wikimedia/wikipedia/commit/b35d406bd9e81f08c68e7bf95d130d2f506dfe77)

  With these improvements, I can generate the English Wikipedia in 5h using 16 processors in a machine without needing a huge amount of RAM (the machine had 32 GB, but I think less can be used as well):
  ```python
  ds = load_dataset("wikimedia/wikipedia", revision="script", date="20231101", language="en", host="https://mirror.accum.se/mirror/wikimedia.org/dumps", split="train", num_proc=16)
  ```
- Pre-process all Wikipedia languages for the latest 2023-11-01 dump and make them available to the entire community for easy use:
  ```python
  ds = load_dataset("wikimedia/wikipedia", "20231101.en", split="train", num_proc=16)
  ```
CC: @geohci 
## Adding a Dataset
- **Name:** Add pre-processed data to:
  - *wikimedia/wikipedia*: https://huggingface.co/datasets/wikimedia/wikipedia
  - *wikimedia/wikisource*: https://huggingface.co/datasets/wikimedia/wikisource
- **Description:** Add pre-processed data to the Hub for all languages
- **Paper:** *link to the dataset paper if available*
- **Data:** *link to the Github repository or current dataset location*
- **Motivation:** This will be very useful for the NLP community, as the pre-processing has a high cost for lot of researchers (both in computation and in knowledge)

Instructions to add a new dataset can be found [here](https://github.com/huggingface/datasets/blob/master/ADD_NEW_DATASET.md).

CC: @geohci, @yjernite