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/pushover.md

@@ -0,0 +1,27 @@
+---
+title: Pushover
+description: Configure Pushover notifications.
+sidebar_position: 8
+---
+
+# Pushover
+
+:::info
+Users can optionally configure personal notifications in their user settings.
+
+User notifications are separate from system notifications, and the available notification types are dependent on user permissions.
+:::
+
+## Configuration
+
+### Application/API Token
+
+[Register an application](https://pushover.net/apps/build) and enter the API token in this field. (You can use one of the [official icons in our GitHub repository](https://github.com/fallenbagel/jellyseerr/tree/develop/public) when configuring the application.)
+
+For more details on registering applications or the API token, please see the [Pushover API documentation](https://pushover.net/api#registration).
+
+### User Key
+
+Set this to the user key for your Pushover account. Alternatively, you can set this to a group key to deliver notifications to multiple users.
+
+For more details, please see the [Pushover API documentation](https://pushover.net/api#identifiers).