docs: Add eslint-mdx support

Official guidelines:
https://github.com/mdx-js/eslint-mdx

Steps taken:
npm i -D eslint-plugin-mdx
Edit .eslintrc.js

docs/.eslintrc.js

@@ -8,6 +8,7 @@ module.exports = {
   extends: [
     "eslint:recommended",
     "plugin:react/recommended",
+    "plugin:mdx/recommended",
     "prettier",
     "prettier/react",
   ],

docs/package.json

@@ -30,6 +30,7 @@
   "devDependencies": {
     "eslint": "^7.12.0",
     "eslint-config-prettier": "^6.14.0",
+    "eslint-plugin-mdx": "^1.8.2",
     "eslint-plugin-react": "^7.21.5",
     "prettier": "2.1.2"
   }