`wordpress_azure_ci_cd.md`: Source Integration (CI/CD) via Deployment Center not available for apps provisioned with `sitecontainers` model

## Description
### Summary
The CI/CD documentation at [`WordPress/wordpress_azure_ci_cd.md`](https://github.com/Azure/wordpress-linux-appservice/blob/main/WordPress/wordpress_azure_ci_cd.md) states that GitHub source integration is available for WordPress images using PHP 8.2 or greater. However, it does not mention a critical limitation: apps provisioned using the newer **SiteContainers** model (`linuxFxVersion = "sitecontainers"`) do **not** have the Source Integration UI available in the Azure Portal Deployment Center.

### Observed Behavior
When a WordPress app is provisioned from Azure Marketplace and the resulting app has:
```json
"linuxFxVersion": "sitecontainers",
```

The Deployment Center blade shows only the **Containers / Logs / FTPS Credentials** tabs. The **Source** tab (GitHub / Azure Repos integration) is absent entirely.
This affects apps provisioned more recently via the Marketplace WordPress offering, regardless of the PHP version or container image tag used (e.g., `mcr.microsoft.com/appsvc/wordpress-debian-php:8.4`).

### Expected Behavior
The documentation should:
1. **Clarify** that the portal-based source integration setup described in `wordpress_azure_ci_cd.md` is only applicable to apps where `linuxFxVersion` is **not** `sitecontainers` (e.g., `WORDPRESS|8.x`).
2. **Document the workaround** for `sitecontainers`-based apps: CI/CD via GitHub Actions is still possible by manually setting up a workflow, as described in [Manually set up a GitHub Actions workflow](https://learn.microsoft.com/en-us/azure/app-service/deploy-github-actions?tabs=openid%2Caspnetcore#manually-set-up-a-github-actions-workflow).
3. **Explain how to identify** which provisioning model an app is using (e.g., check `linuxFxVersion` via Azure CLI or Portal → JSON View).

### Steps to Reproduce
1. Create a new WordPress app via Azure Marketplace.
2. Navigate to **Deployment Center** in the Azure Portal.
3. Observe that only **Containers / Logs / FTPS Credentials** tabs are shown — no Source tab.
4. Confirm `linuxFxVersion` = `sitecontainers` via:
```
az webapp config show --name --resource-group --query linuxFxVersion
```

### Suggested Documentation Addition
Add a note such as:
> **Note:** If your app was provisioned using the SiteContainers model (`linuxFxVersion = "sitecontainers"`), the source integration option will not appear in Deployment Center. In this case, you can still set up GitHub Actions CI/CD manually. See [Manually set up a GitHub Actions workflow](https://learn.microsoft.com/en-us/azure/app-service/deploy-github-actions?tabs=openid%2Caspnetcore#manually-set-up-a-github-actions-workflow).