Update docusaurus to 2.0.0-beta.0

docs/package.json

@@ -14,9 +14,9 @@
     "write-heading-ids": "docusaurus write-heading-ids"
   },
   "dependencies": {
-    "@docusaurus/core": "2.0.0-alpha.72",
-    "@docusaurus/plugin-ideal-image": "2.0.0-alpha.72",
-    "@docusaurus/preset-classic": "2.0.0-alpha.72",
+    "@docusaurus/core": "2.0.0-beta.0",
+    "@docusaurus/plugin-ideal-image": "2.0.0-beta.0",
+    "@docusaurus/preset-classic": "2.0.0-beta.0",
     "@mdx-js/react": "^1.6.21",
     "autoprefixer": "^10.2.5",
     "clsx": "^1.1.1",
@@ -43,6 +43,7 @@
   },
   "devDependencies": {
     "postcss-import": "^14.0.1",
+    "postcss-preset-env": "^6.7.0",
     "tailwindcss": "^2.1.2"
   }
 }