added JS file (#2387)

R-package/.Rbuildignore

 ^build_package.R$
 \.gitkeep$
 ^docs$
-^_pkgdown\.yml$
+^pkgdown$
 
 # Objects created by compilation
 \.o$

R-package/pkgdown/extra.js

+$(function() {
+    if(window.location.pathname.toLocaleLowerCase().indexOf('/r/reference') != -1) {
+        /* Replace '/R/' with '/R-package/R/' in all external links to .R files of LightGBM GitHub repo */
+        $('a[href^="https://github.com/Microsoft/LightGBM/blob/master/R"][href*=".R"]').attr('href', (i, val) => { return val.replace('/R/', '/R-package/R/'); });
+    }
+});