docs: added slack integration guide (#12696)

* docs: added slack integration guide

* generate

www/apps/resources/sidebars/integrations.mjs

@@ -122,6 +122,11 @@ export const integrationsSidebar = [
         path: "/infrastructure-modules/notification/sendgrid",
         title: "SendGrid",
       },
+      {
+        type: "link",
+        path: "/integrations/guides/slack",
+        title: "Slack",
+      },
       {
         type: "ref",
         path: "/how-to-tutorials/tutorials/phone-auth#step-3-integrate-twilio-sms",