LightGBM

[docs][R] added R-package docs generation routines (#2176)

* added R-package docs generation routines

* change theme to be more consistent with sphinx_rtd_theme on main site in terms of color scheme

* placed man folder with old Rd files back

* specify full path to conda and make script more readable by one line - one pkg

* removed commented lines from build_r_site script

* made one line - one argument in build_reference() call

* pin R package versions

* fixed conflict