chore(eslint): upgrade to eslint v9 and flat config (#2909)

docs/eslint.config.mjs

@@ -0,0 +1,28 @@
+import { defineConfig, globalIgnores } from "eslint/config";
+import globals from "globals";
+import js from "@eslint/js";
+import pluginReact from "eslint-plugin-react";
+import * as mdx from "eslint-plugin-mdx";
+
+export default defineConfig([
+  globalIgnores([".cache-loader/", ".docusaurus/", "build/"]),
+  { files: ["**/*.{js,jsx,md,mdx}"] },
+  {
+    plugins: { js },
+    extends: ["js/recommended"],
+  },
+  mdx.flat,
+  pluginReact.configs.flat.recommended,
+  pluginReact.configs.flat["jsx-runtime"],
+  {
+    languageOptions: {
+      globals: { ...globals.browser, ...globals.commonjs, ...globals.node },
+    },
+    rules: { "react/no-unescaped-entities": "off" },
+    settings: {
+      react: {
+        version: "detect",
+      },
+    },
+  },
+]);