fixed broken links in index.md

doc/user/index.md

@@ -161,13 +161,13 @@ such as Trello, JIRA, etc.
 
 ## Webhooks
 
-Configure [webhooks](project/integrations/webhooks.html) to listen for
+Configure [webhooks](project/integrations/webhooks.md) to listen for
 specific events like pushes, issues or merge requests. GitLab will send a
 POST request with data to the webhook URL.
 
 ## API
 
-Automate GitLab via [API](../api/README.html).
+Automate GitLab via [API](../api/README.md).
 
 ## Git and GitLab