Update installation instructions in documentation

- Added detailed steps for saving a sample test file as _sample.spec.json_.
- Updated the command to run a test with the correct input file.

Author: hawkeyexl

docs/get-started/installation.md

@@ -18,10 +18,38 @@ Downloading and running Doc Detective is straightforward. Follow these steps to
 
    **Note:** If you're working in a local copy of the `doc-detective` repository, you need to run `npm i` to install local dependencies.
 
-3. Run a test:
+3. Save this file to your current directory as _sample.spec.json_:
+
+   ```json
+   {
+     "tests": [
+       {
+         "steps": [
+           {
+             "description": "Go to the specified URL",
+             "goTo": "https://example.com"
+           },
+           {
+             "description": "Verify the presence of the main heading",
+             "find": "Example Domain"
+           },
+           {
+             "description": "Verify that the 'More information...' link is present and working",
+             "click": "More information..."
+           },
+           {
+             "description": "Capture a screenshot of the resulting page",
+             "screenshot": "example.png"
+           }
+         ]
+       }
+     ]
+   }   ```
+
+4. Run a test, pointing to the file you just created:
 
    ```bash
-   npx doc-detective runTests --input https://doc-detective.com/sample.spec.json
+   npx doc-detective --input sample.spec.json
    ```
 
 Congratulations! You've run your first test with Doc Detective.