code: def build_r_docs(app):
      
      r_root = app.builder.srcdir + '/../R-package'
      pdf_path = app.builder.srcdir + '/api/r/mxnet-r-reference-manual.pdf'
      _run_cmd('cd ' + r_root +
               '; R -e "roxygen2::roxygenize()"; R CMD Rd2pdf . --no-preview -o ' + pdf_path)
      dest_path = app.builder.outdir + '/api/r/'
      _run_cmd('mkdir -p ' + dest_path + '; mv ' + pdf_path + ' ' + dest_path)
docs: build r pdf