chore: update GH's template and prettier config (#309)

* chore(gh): update GitHub's pull-request template

* chore: use unified common prettier config

* chore: add the mdx file to prettier

* chore(gh): update contributing guide docs

package.json

@@ -15,6 +15,7 @@
     "type": "git",
     "url": "https://github.com/zeit-ui/react"
   },
+  "prettier": "@zeit-ui/prettier-config",
   "keywords": [
     "zeit",
     "zeit ui",
@@ -65,6 +66,7 @@
     "@types/styled-jsx": "^2.2.8",
     "@typescript-eslint/eslint-plugin": "^2.24.0",
     "@typescript-eslint/parser": "^2.24.0",
+    "@zeit-ui/prettier-config": "^1.0.0",
     "@zeit-ui/react-icons": "^1.2.2",
     "babel-jest": "^25.3.0",
     "babel-loader": "^8.0.6",