Move DatasetInfo from `datasets_infos.json` to the YAML tags in `README.md`
Currently there are two places to find metadata for datasets:
- datasets_infos.json, which contains **per dataset config**
  - description
  - citation
  - license
  - splits and sizes
  - checksums of the data files
  - feature types
  - and more
- YAML tags, which contain
  - license
  - language
  - train-eval-index
  - and more

It would be nice to have a single place instead. We can rely on the YAML tags more than the JSON file for consistency with models. And it would all be indexed by our back-end directly, which is nice to have. 

One way would be to move everything to the YAML tags except the checksums (there can be tens of thousands of them). The description/citation is already in the dataset card so we probably don't need to have them in the YAML card, it would be redundant.

Here is an example for SQuAD
```yaml

download_size: 35142551
dataset_size: 89789763
version: 1.0.0
splits:
- name: train
  num_examples: 87599
  num_bytes: 79317110
- name: validation
  num_examples: 10570
  num_bytes: 10472653
features:
- name: id
  dtype: string
- name: title
  dtype: string
- name: context
  dtype: string
- name: question
  dtype: string
- name: answers
  struct:
  - name: text
    list:
      dtype: string
  - name: answer_start
    list:
      dtype: int32
```

Since there is only one configuration for SQuAD, this structure is ok. For datasets with several configs we can see in a second step, but IMO it would be ok to have these fields per config using another syntax
```yaml
configs:
- config: unlabeled
  splits:
  - name: train
    num_examples: 10000
  features:
  - name: text
    dtype: string
- config: labeled
  splits:
  - name: train
    num_examples: 100
  features:
  - name: text
    dtype: string
  - name: label
    dtype: ClassLabel
    names:
    - negative
    - positive
```
So in the end you could specify a YAML tag either at the top level (for all configs) or per config in the `configs` field

Alternatively we could keep config specific stuff in the `dataset_infos.json` as it it today

Not sure yet what's the best approach here but cc @julien-c @mariosasko @albertvillanova @polinaeterna for feedback :)