Docs: Clarify how to use local file paths with CLI

Existing documentation focused on using the CLI with URLs
(e.g. `hint https://localhost:8080`). This change adds an example
showing usage with filesystem paths (e.g. `hint ./path/to/my.html`)

- - - - - - - - - - - - - - - - - - - -

Close #4256

packages/hint/docs/user-guide/index.md

@@ -29,6 +29,12 @@ best practices that may be applied to your site, complete the following steps.
    npx hint https://example.com
    ```
 
+   or
+
+   ```bash
+   npx hint ./path/to/my.html
+   ```
+
 1. After the `webhint` process completes, a summary is presented in the bash
    command-line interface with a link to a report file on your computer.
 1. Navigate to the report file and open it. The report file outlines the