Add a section for the missing endpoints in the doc
Missing documentation:
- [x] /size (dataset, config)
- [x] /info (dataset, config)
- [x] /statistics (split)
- [x] /search (split) - see #1663
- [ ] /opt-in-out-urls (dataset, config, split)

Add a section for the missing endpoints in the doc: Missing documentation:
- [x] /size (dataset, config)
- [x] /info (dataset, config)
- [x] /statistics (split)
- [x] /search (split) - see #1663
- [ ] /opt-in-out-urls (dataset, config, split)
