GitLab

* [R-package] [docs] Reorganize installation instructions (fixes #3320)

* more changes

* remove ability to use precompiled lib_lightgbm

* remove language about installing from the CRAN section

* move installation stuff

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

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