GitLab

* [R-package] introduce Dataset set_field() and get_field()

* fix incorrect fields

* update pkgdown

* fix example

* fix another example

* Apply suggestions from code review
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* update docs
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>