Docs: add example .eslintrc.json (#15087)

* Docs: add example .eslintrc.json

This adds a `.eslintrc.json` that shows how to set rules config (among
others).

* Update docs/user-guide/configuring/configuration-files.md

Co-authored-by: Nicholas C. Zakas <nicholas@humanwhocodes.com>

* Update docs/user-guide/configuring/configuration-files.md

Co-authored-by: Nicholas C. Zakas <nicholas@humanwhocodes.com>

* Update docs/user-guide/configuring/configuration-files.md

Co-authored-by: Nicholas C. Zakas <nicholas@humanwhocodes.com>

Co-authored-by: Nicholas C. Zakas <nicholas@humanwhocodes.com>

Author: nmattia

docs/user-guide/configuring/configuration-files.md

@@ -39,6 +39,33 @@ The second way to use configuration files is to save the file wherever you would
 
 If you are using one configuration file and want ESLint to ignore any `.eslintrc.*` files, make sure to use [`--no-eslintrc`](https://eslint.org/docs/user-guide/command-line-interface#--no-eslintrc) along with the [`-c`](https://eslint.org/docs/user-guide/command-line-interface#-c---config) flag.
 
+Here's an example JSON configuration file that uses the `typescript-eslint` parser to support TypeScript syntax:
+
+```json
+{
+    "root": true,
+    "extends": [
+        "eslint:recommended",
+        "plugin:@typescript-eslint/recommended"
+    ],
+    "parser": "@typescript-eslint/parser",
+    "parserOptions": { "project": ["./tsconfig.json"] },
+    "plugins": [
+        "@typescript-eslint"
+    ],
+    "rules": {
+        "@typescript-eslint/strict-boolean-expressions": [
+            2,
+            {
+                "allowString" : false,
+                "allowNumber" : false
+            }
+        ]
+    },
+    "ignorePatterns": ["src/**/*.test.ts", "src/frontend/generated/*"]
+}
+```
+
 ### Comments in configuration files
 
 Both the JSON and YAML configuration file formats support comments (package.json files should not include them). You can use JavaScript-style comments for JSON files and YAML-style comments for YAML files. ESLint safely ignores comments in configuration files. This allows your configuration files to be more human-friendly.