Replace valid with preview and viewer
replaces #1450 and #1447

fixes #1446 and #1445

- [x] remove `valid` field from `/valid` endpoint
- [x] replace `valid` with `viewer` and `preview` in `/is-valid`
- [x] update the docs
- [x] update openapi, <strike>rapidapi</strike> (I don't really understand rapidapi anymore), postman

This change is breaking the API for /valid and /is-valid. See https://moon-ci-docs.huggingface.co/docs/datasets-server/pr_1452/en/valid.

- [x] for Autotrain, I think the endpoints are not used in the code (https://github.com/huggingface/autotrain-ui and https://github.com/huggingface/autotrain-backend/). cc @abhishekkrthakur 
- [x] for model-evaluator, I opened https://github.com/huggingface/model-evaluator/pull/71 cc @lewtun 
Replace valid with preview and viewer: replaces #1450 and #1447

fixes #1446 and #1445

- [x] remove `valid` field from `/valid` endpoint
- [x] replace `valid` with `viewer` and `preview` in `/is-valid`
- [x] update the docs
- [x] update openapi, <strike>rapidapi</strike> (I don't really understand rapidapi anymore), postman

This change is breaking the API for /valid and /is-valid. See https://moon-ci-docs.huggingface.co/docs/datasets-server/pr_1452/en/valid.

- [x] for Autotrain, I think the endpoints are not used in the code (https://github.com/huggingface/autotrain-ui and https://github.com/huggingface/autotrain-backend/). cc @abhishekkrthakur 
- [x] for model-evaluator, I opened https://github.com/huggingface/model-evaluator/pull/71 cc @lewtun 