Dataset infos in yaml
To simplify the addition of new datasets, we'd like to have the dataset infos in the YAML and deprecate the dataset_infos.json file. YAML is readable and easy to edit, and the YAML metadata of the readme already contain dataset metadata so we would have everything in one place.

To be more specific, I moved these fields from DatasetInfo to the YAML:
- config_name (if there are several configs)
- download_size
- dataset_size
- features
- splits

Here is what I ended up with for `squad`:
```yaml
dataset_info:
  features:
  - name: id
    dtype: string
  - name: title
    dtype: string
  - name: context
    dtype: string
  - name: question
    dtype: string
  - name: answers
    sequence:
    - name: text
      dtype: string
    - name: answer_start
      dtype: int32
  splits:
  - name: train
    num_bytes: 79346360
    num_examples: 87599
  - name: validation
    num_bytes: 10473040
    num_examples: 10570
  config_name: plain_text
  download_size: 35142551
  dataset_size: 89819400
```

and it can be a list if there are several configs

I already did the change for `conll2000` and `crime_and_punish` as an example.

## Implementation details

### Load/Read

This is done via `DatasetInfosDict.write_to_directory/from_directory`

I had to implement a custom the YAML export logic for `SplitDict`, `Version` and `Features`.
The first two are trivial, but the logic for `Features` is more complicated, because I added a simplification step (or the YAML would be too long and less readable): it's just a formatting step to remove unnecessary nesting of YAML data.

### Other changes

I had to update the DatasetModule factories to also download the README.md alongside the dataset scripts/data files, and not just the dataset_infos.json

## YAML validation

I removed the old validation code that was in metadata.py, now we can just use the Hub YAML validation

## Datasets-cli

The `datasets-cli test --save_infos` command now creates a README.md file with the dataset_infos in it, instead of a datasets_infos.json file

## Backward compatibility

`dataset_infos.json` files are still supported and loaded if they exist to have full backward compatibility.
Though I removed the unnecessary keys when the value is the default (like all the `id: null` from the Value feature types) to make them easier to read.

## TODO

- [x] add comments
- [x] tests
- [x] document the new YAML fields 
- [x] try to reload the new dataset_infos.json file content with an old version of `datasets`

## EDITS

- removed "config_name" when there's only one config
- removed "version" for now (?), because it's not useful in general
- renamed the yaml field "dataset_info" instead of "dataset_infos", since it only has one by default (and because "infos" is not english)

Fix https://github.com/huggingface/datasets/issues/4876 and fix #2773