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/.eslintignore

@@ -0,0 +1,2 @@
+node_modules
+build

docs/.eslintrc.js

@@ -0,0 +1,23 @@
+module.exports = {
+  env: {
+    browser: true,
+    commonjs: true,
+    es2021: true,
+    node: true,
+  },
+  extends: ["eslint:recommended", "plugin:react/recommended"],
+  parserOptions: {
+    ecmaFeatures: {
+      jsx: true,
+    },
+    ecmaVersion: 2021,
+    sourceType: "module",
+  },
+  plugins: ["react"],
+  rules: {},
+  settings: {
+    react: {
+      version: "detect",
+    },
+  },
+};

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"
   }
 }