help: Document Do Not Disturb feature in Desktop app.

Documents the Do Not Disturb option with a dedicated page in
the Notifications section.

The capitalization is not our standard style because the desktop app
doesn't follow that style convention.

Fixes #24169.

help/do-not-disturb.md

@@ -0,0 +1,56 @@
+# Do Not Disturb
+
+The Zulip desktop app offers a **Do Not Disturb (DND)** mode. Turning on **Do Not
+Disturb** mode disables [desktop notifications](/help/desktop-notifications)
+across all the organizations you have
+[added](/help/logging-in#log-in-for-the-first-time) to the Zulip desktop app.
+This can be convenient for avoiding interruptions during a meeting, or when you need
+time to focus on your work.
+
+
+!!! warn ""
+
+    Other types of notifications will not be affected, including [mobile
+    notifications](/help/mobile-notifications), [email
+    notifications](/help/email-notifications), and [desktop
+    notifications](/help/desktop-notifications) in the Zulip web app.
+
+## Toggle Do Not Disturb
+
+{start_tabs}
+
+{tab|desktop}
+
+1. Click the **bell** (<i class="fa fa-bell"></i>) or **bell with a slash** (<i
+   class="fa fa-bell-slash"></i>) icon in the **organizations sidebar** on the
+   left. You can also select **Toggle Do Not Disturb** from the **Zulip** menu in
+   the top menu bar.
+
+{!desktop-toggle-sidebar-tip.md!}
+
+{end_tabs}
+
+## Check whether Do Not Disturb is enabled
+
+{start_tabs}
+
+{tab|desktop}
+
+1. If the **organizations sidebar** on the left shows a **bell** (<i class="fa
+   fa-bell"></i>) icon, **Do Not Disturb** is disabled. If the **organizations
+   sidebar** on the left shows a **bell with a slash** (<i class="fa
+   fa-bell-slash"></i>) icon, **Do Not Disturb** is enabled, and you are not
+   receiving desktop notifications.
+
+{!desktop-toggle-sidebar-tip.md!}
+
+{end_tabs}
+
+
+## Related articles
+
+* [Desktop notifications](/help/desktop-notifications)
+* [PMs, mentions, and alerts](/help/pm-mention-alert-notifications)
+* [Stream notifications](/help/stream-notifications)
+* [Email notifications](/help/email-notifications)
+* [Mobile notifications](/help/mobile-notifications)