docs: notification agent docs added

docs/using-overseerr/notifications/README.md

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+# Notifications
+
+Overseerr already supports a good number of notification agents, such as **Discord**, **Slack** and **Pushover**. New agents are always considered for development, if there is enough demand for it.
+
+## Currently Supported Notification Agents
+
+- Email
+- Discord
+- Slack
+- Telegram
+- Pushover
+- [Webhooks](./webhooks.md)
+
+## Setting up Notifications
+
+Configuring your notifications is _very simple_. First, you will need to visit the **Settings** page and click **Notifications** in the menu. This will present you with all of the currently available notification agents. Click on each one individually to configure them.
+
+You must configure which type of notifications you want to send _per agent_. If no types are selected, you will not receive any notifications!
+
+Some agents may have specific configuration gotchas that will be covered in each notification agents documentation page.
+
+{% hint style="danger" %}
+Currently, you will **not receive notifications** for any auto-approved requests. However, you will still receive a notification when the media becomes available.
+{% endhint %}
+
+## Requesting new agents
+
+If we do not currently support a notification agent you would like, feel free to request it on our [GitHub Issues](https://github.com/sct/overseerr/issues). Make sure to search first to see if someone else already requested it!