build r pdf

def build_r_docs(app):
    """build r pdf"""
    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)