Add docs about datalist fields

- [ ] BooleanFieldType
- [ ] DateTimeFieldType
- [ ] HeadingFieldType (may be removed in next major, let's see in the future)
- [ ] ImageFieldType
- [x] LabelFieldType
- [ ] TextFieldType
- [ ] UrlFieldType
- [ ] Common docs about fields (label, callback...)
- [ ] Create your own field