[docs] refer to RTD site directly (#2415)

docs/.linkcheckerrc

@@ -8,7 +8,6 @@ ignore=
   public.tableau.com
   https://www.open-mpi.org
   https://readthedocs.org
-  .*R/reference$
 ignorewarnings=http-robots-denied,https-certificate-error
 
 [output]

docs/R-API.rst

-R API
-=====
-
-Refer to `R reference <./R/reference>`__.

docs/index.rst

@@ -29,7 +29,7 @@ For more details, please refer to `Features <./Features.rst>`__.
    Parameters Tuning <Parameters-Tuning>
    C API <C-API>
    Python API <Python-API>
-   R API <R-API>
+   R API <https://lightgbm.readthedocs.io/en/latest/R/reference/>
    Parallel Learning Guide <Parallel-Learning-Guide>
    GPU Tutorial <GPU-Tutorial>
    Advanced Topics <Advanced-Topics>