docs: Add eslint support

Official guidelines:
https://eslint.org/docs/user-guide/getting-started

Steps taken:
npm install eslint --save-dev
npx eslint --init
Create .eslintignore
Edit .eslintrc.js (tweaks)

docs/package.json

@@ -26,5 +26,9 @@
       "last 1 firefox version",
       "last 1 safari version"
     ]
+  },
+  "devDependencies": {
+    "eslint": "^7.12.0",
+    "eslint-plugin-react": "^7.21.5"
   }
 }