docs(notifications): add more documentation for notifications (#1856)

This PR adds more documentation methods for notifications. Most of it is taken from the Overseerr
documentation.

docs/using-jellyseerr/notifications/slack.md

@@ -0,0 +1,17 @@
+---
+title: Slack
+description: Configure Slack notifications.
+sidebar_position: 9
+---
+
+# Slack
+
+## Configuration
+
+### Webhook URL
+
+Simply [create a webhook](https://my.slack.com/services/new/incoming-webhook/) and enter the URL in this field.
+
+:::info
+Please refer to the [Slack API documentation](https://api.slack.com/messaging/webhooks) for more details on configuring these notifications.
+:::