feat: add user specific notification settings

The settings can be customized for each origin and each processor
individually. The default Web indicates notifications shown in the
Backstage UI. By default, if there are no settings saved in the
database, all notifications are enabled for all processors.

The origins will populate by time for each user as they receive the
first notification from that origin. Processors are shown as their own
columns.

Later, if it makes sense, allow users to also disable/enable
notifications based on notification topic.

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

docs/notifications/index.md

@@ -350,6 +350,31 @@ export const myPlugin = createBackendPlugin({
 });
 ```
 
+### User-specific notification settings
+
+The notifications plugin provides a way for users to manage their notification settings. To enable this, you must
+add the `UserNotificationSettingsCard` to your frontend.
+
+```tsx
+// App.tsx example
+<Route path="/settings" element={<UserSettingsPage />}>
+  <SettingsLayout.Route path="/advanced" title="Advanced">
+    <AdvancedSettings />
+  </SettingsLayout.Route>
+  <SettingsLayout.Route path="/notifications" title="Notifications">
+    <UserNotificationSettingsCard
+      originNames={{ 'plugin:scaffolder': 'Scaffolder' }}
+    />
+  </SettingsLayout.Route>
+</Route>
+```
+
+![Notification Settings](notificationSettings.png)
+
+You can customize the origin names shown in the UI by passing an object where the keys are the origins and the values are the names you want to show in the UI.
+
+Each notification processor will receive its own column in the settings page, where the user can enable or disable notifications from that processor.
+
 ### External Services
 
 When the emitter of a notification is a Backstage backend plugin, it is mandatory to use the integration via `@backstage/plugin-notifications-node` as described above.