feat: add slack notification guide

docs/notifications.md

@@ -41,6 +41,25 @@ Here is an example:
 
 ![OpenCVE Email Notification](images/opencve-email-notification.png){.center style="width:100%"}
 
+### Slack
+
+Slack notifications allow you to share CVE updates directly in your Slack channels, making it easy to keep your team informed about security vulnerabilities.
+
+To configure Slack notifications, you need to create an incoming webhook in your Slack workspace by following the [official documentation](https://docs.slack.dev/messaging/sending-messages-using-incoming-webhooks/).
+
+This will generate a webhook URL in the format:
+```
+https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX
+```
+
+Simply provide this URL in the notification configuration to start receiving CVE updates in your chosen Slack channel:
+
+![OpenCVE Slack Notification](images/opencve-slack-notification-form.png){.center style="width:70%"}
+
+Here is an example:
+
+![OpenCVE Slack Notification](images/opencve-slack-notification.png){.center style="width:100%"}
+
 ### Webhook
 
 Webhook notifications allow users to interface with their own information systems or tools, such as an ITSM service.