docs: Migrate to Flat Config (#302)

docs/use/migrate-to-flat-config.md

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+# Migrate to Flat Config
+
+Starting with ESLint v8.21.0, a new configuration method, Flat Config, has been officially introduced. This new format aims to reduce the complexity of configuration files and make them more intuitive and understandable. And eslint-config-moneyforward also officially supports Flat Config from v4.0.0. This document explains how to migrate from the old eslintrc format to the Flat Config format.
+
+## Step1: Understand your existing configuration
+
+First, review your existing the `.eslintrc.*` file and understand the rules, environment settings, etc. based on eslint-config-moneyforward.
+
+```js
+// .eslintrc.js
+module.exports = {
+  extends: [
+    'moneyforward/essentials',
+    'moneyforward/next',
+    'moneyforward/react',
+    'moneyforward/typescript',
+    'moneyforward/test/react',
+    'prettier',
+  ],
+  rules: {
+    'no-bitwise': ['off'],
+    'no-plusplus': ['off'],
+  },
+  overrides: [
+    {
+      files: ['**/*.@(ts|tsx)'],
+      rules: {
+        '@typescript-eslint/no-throw-literal': ['off'],
+      },
+    },
+  ],
+};
+```
+
+## Step2: Create a new configuration file
+
+Flat Config uses a new file name `eslint.config.js` (or `eslint.config.mjs`), instead of `.eslintrc.*`. Create this file in the same directory as the existing `.eslintrc.*`.
+
+## Step3: Write the settings in Flat Config format
+
+Write the following settings in the created `eslint.config.js`.
+
+```js
+// eslint.config.js
+import {
+  essentials,
+  next,
+  react,
+  test,
+  typescript,
+} from 'eslint-config-moneyforward/flat';
+import prettier from 'eslint-config-prettier';
+
+export default [
+  ...essentials,
+  ...next,
+  ...react,
+  ...typescript,
+  ...test.react,
+
+  {
+    rules: {
+      'no-bitwise': ['off'],
+      'no-plusplus': ['off'],
+    },
+  },
+  {
+    files: ['**/*.@(ts|tsx)'],
+    rules: {
+      '@typescript-eslint/no-throw-literal': ['off'],
+    },
+  },
+
+  prettier,
+];
+```
+
+## Step4: Delete the `.eslintrc.*` file
+
+After you finish migrating to Flat Config, delete the existing `.eslintrc.*`.
+
+## Step5: Test and Tuning
+
+Run ESLint to test that the newly created configuration works currently. If any problems occur, tune and adjust the configuration file to resolve the issue.
+
+## References
+
+- [Configuration Migration Guide - ESLint - Pluggable JavaScript Linter](https://eslint.org/docs/latest/use/configure/migration-guide)