Add example of using environment variables with GitHub Pages deployment

src/content/docs/en/guides/deploy/github.mdx

@@ -138,11 +138,28 @@ export default defineConfig({
     The official Astro [action](https://github.com/withastro/action) scans for a lockfile to detect your preferred package manager (`npm`, `yarn`, `pnpm`, or `bun`). You should commit your package manager's automatically generated `package-lock.json`, `yarn.lock`, `pnpm-lock.yaml`, or `bun.lockb` file to your repository.
     :::
 
-2. On GitHub, go to your repository’s **Settings** tab and find the **Pages** section of the settings.
+2. (Optional) If you pass environment variables to your Astro project during local development or when previewing builds, you will need to define any public variables in the `deploy.yml` file in order for them to be processed when you deploy to GitHub Pages. (See the [GitHub documentation on setting secrets](https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/store-information-in-variables#creating-configuration-variables-for-a-repository) for adding private environment variables.)
 
-3. Choose **GitHub Actions** as the **Source** of your site.  
+   ```yaml title="deploy.yml"
+   jobs:
+      build:
+        runs-on: ubuntu-latest
+        steps:
+          - name: Checkout your repository using git
+            uses: actions/checkout@v4
+          - name: Install, build, and upload your site
+            uses: withastro/action@v3
+            env:
+              # Use single quotation marks for the variable value
+              PUBLIC_EVM_WALLET_ADDRESS: '0x4bFc229A40d41698154336aFF864f61083E76659'
+   ```
+
+
+3. On GitHub, go to your repository’s **Settings** tab and find the **Pages** section of the settings.
+
+4. Choose **GitHub Actions** as the **Source** of your site.  
 
-4. Commit the new workflow file and push it to GitHub.  
+5. Commit the new workflow file and push it to GitHub.  
 </Steps>
 
 Your site should now be published! When you push changes to your Astro project’s repository, the GitHub Action will automatically deploy them for you.