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* [R-package] [docs] add intro vignette (#3946)

* add 10 test vignettes

* Revert "add 10 test vignettes"

This reverts commit 40fb2e2f1982402798776ee44e4ec82fc4644d3d.

* Apply suggestions from code review
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>
Co-authored-by: Michael Mayer <mayermichael79@gmail.com>
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>