docs: add Pushbullet, Pushover, Slack, Telegram (#1268) [skip ci]

* docs: add Pushbullet, Pushover, Telegram

* docs: correction based on feedback from @costaht

* docs: add Slack

* docs: update Telegram docs to reflect changes in #1172

* docs: clarify PGP key format

* docs: edit Pushover documentation

docs/using-overseerr/notifications/pushover.md

@@ -0,0 +1,15 @@
+# Pushover
+
+## 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/sct/overseerr/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).