[R-package] [docs] replace build_r_site.R with pkgdown::build_site() (#3098)

* [R-package] [docs] replace build_r_site.R with pkgdown::build_site()

* fix paths

* fix continuation

* new_process arg

build_r_site.R

-library(pkgdown)
-library(roxygen2)
-
-setwd("lightgbm_r")
-if (!dir.exists("docs")) {
-  dir.create("docs")
-}
-
-roxygen2::roxygenize(load = "installed")
-pkgdown::clean_site()
-pkgdown::init_site()
-pkgdown::build_home(preview = FALSE, quiet = FALSE)
-pkgdown::build_reference(
-    lazy = FALSE
-    , devel = FALSE
-    , examples = TRUE
-    , run_dont_run = FALSE
-    , seed = 42L
-    , preview = FALSE
-)

docs/conf.py

@@ -240,8 +240,21 @@ def generate_r_docs(app):
     cd {0}
     export R_LIBS="$CONDA_PREFIX/lib/R/library"
     Rscript build_r.R
-    Rscript build_r_site.R
-    """.format(os.path.join(CURR_PATH, os.path.pardir))
+    cd {1}
+    Rscript -e "roxygen2::roxygenize(load = 'installed')"
+    Rscript -e "pkgdown::build_site( \
+            lazy = FALSE \
+            , install = FALSE \
+            , devel = FALSE \
+            , examples = TRUE \
+            , run_dont_run = FALSE \
+            , seed = 42L \
+            , preview = FALSE \
+            , new_process = TRUE \
+        ) \
+        "
+    cd {0}
+    """.format(os.path.join(CURR_PATH, os.path.pardir), os.path.join(CURR_PATH, os.path.pardir, "lightgbm_r"))
     try:
         # Warning! The following code can cause buffer overflows on RTD.
         # Consider suppressing output completely if RTD project silently fails.