Support Dataset conversion from/to Spark
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5401). All of your documentation changes will be reflected on that endpoint. Cool thanks !

Spark DataFrame are usually quite big, and I believe here `from_spark` would load everything in the driver node's RAM, which is quite limiting. Same for `to_spark` which would load everything in the driver node's RAM before sending the data to the executor. Maybe we can mention this in the docstring ?

To transfer big datasets from/into the HF ecosystem using Spark maybe we can just make sure that `pyspark` can read/write to the HF Hub, and that `datasets` can read from HDFS/S3/etc. Yes @lhoestq , consider this as a first integration of the Datasets library with Spark.
- This PR implements the basic conversion between both.
  - And yes, we are using the Spark's `pandas` API (that uses `pyarrow` under the hood): everything is transferred to the driver.
  - Note that we are converting from/to a Datasets dataset: this is not distributed

The next step is to support the integration of the HF Hub with Spark, that I think should be done using `hffs`. Thinking more about it I don't really see how those two methods help in practice, since one can already do `datasets` <-> pandas <-> spark and those two methods don't add value over this.

However I think it can be good documentation to explain that it's possible to do it and it's super simple
This PR implements Spark integration by supporting `Dataset` conversion from/to Spark `DataFrame`.