feat: enable tailwind (tree-shaking) for *.mdx files (planning#874)

tailwind.config.js

 module.exports = {
   mode: 'jit',
-  purge: ['src/**/*.js', 'src/**/*.jsx', 'src/**/*.ts', 'src/**/*.tsx'],
+  purge: ['src/**/*.js', 'src/**/*.jsx', 'src/**/*.ts', 'src/**/*.tsx', 'docs/**/*.mdx'],
   corePlugins: {
     preflight: false,
   },