message: use next-page-map to get page mappings
diff: const withPlugins = require('next-compose-plugins')
  const mdx = require('@zeit/next-mdx')
  +const {join} = require('path')
  +const getPageMap = require('next-page-map')
  +
  +const pageExtensions = ['js', 'jsx', 'md', 'mdx']
  +const pageMap = getPageMap(join(__dirname, 'pages'), pageExtensions)
  module.exports = withPlugins([
  mdx({extension: /\.mdx?$/})
  @@ -10,9 +15,9 @@ module.exports = withPlugins([
  * <https://primer.style/components>
  */
  assetPrefix: process.env.NOW_URL,
  - pageExtensions: ['js', 'jsx', 'md', 'mdx'],
  - sassLoaderOptions: {
  - includePaths: ['node_modules']
  + pageExtensions,
  + publicRuntimeConfig: {
  + pageMap
  },
  webpack(config) {
  