Merge pull request #833 from KnechtionsCoding/main

docs: updating slack alerting docs to point to legacy slack webhooks

docs/gitbook/usage/alerting.md

@@ -5,7 +5,17 @@ You can define a global alert provider at install time or configure alerts on a
 
 ## Global configuration
 
-Flagger can be configured to send Slack notifications:
+### Slack
+
+#### Slack Configuration
+
+Flagger requires a custom webhook integration from slack, instead of the new slack app system.
+
+The webhook can be generated by following the [legacy slack documentation](https://api.slack.com/legacy/custom-integrations/messaging/webhooks)
+
+#### Flagger configuration
+
+Once the webhook has been generated. Flagger can be configured to send Slack notifications:
 
 ```bash
 helm upgrade -i flagger flagger/flagger \
@@ -25,6 +35,8 @@ or if the analysis reached the maximum number of failed checks:
 
 ![Slack Notifications](https://raw.githubusercontent.com/fluxcd/flagger/main/docs/screens/slack-canary-failed.png)
 
+### Microsoft Teams
+
 Flagger can be configured to send notifications to Microsoft Teams:
 
 ```bash