Add default handmade test step using Codespace to new pull requests.

[vitormattos]

Solution description

Now we have a pull request template here:
https://github.com/LibreSign/libresign/blob/main/.github/pull_request_template.md

But would be good to have the follow code at the end of pull request template:

<details>
<summary>How to see this running using GitHub Codespaces</summary>

### 1. Open the Codespace
- Authenticate to GitHub
- Go to the branch: [chore/reduce-configure-check-time](https://github.com/LibreSign/libresign/tree/chore/reduce-configure-check-time)
- Click the `Code` button and select the `Codespaces` tab.
- Click **"Create codespace on feat/customize-signature-stamp"**

### 2. Wait for the environment to start
- A progress bar will appear on the left.  
- After that, the terminal will show the build process.
- Wait until you see the message:  
  ```bash
  ✍️ LibreSign is up!
  ```
  This may take a few minutes.

### 3. Access LibreSign in the browser
- Open the **Ports** tab (next to the **Terminal**).
- Look for the service running on port **80**.
- Hover over the URL and click the **globe icon** 🌐 to open it in your browser.

### 4. (Optional) Make the service public
- If you want to share the app with people **not logged in to GitHub**, you must change the port visibility:
  - Click the three dots `⋮` on the row for port 80.
  - Select `Change visibility` → `Public`.

### 5. Login credentials
- **Username**: `admin`  
- **Password**: `admin`

Done! 🎉
You're now ready to test this.
<details>

Additional context

Look a pull request with this piece of code:

• fix: change display name #5139

This issue is open to contributors. If you're interested, feel free to leave a comment or open a draft PR to get started!