docs: Add "View Zulip version" help center page.

Fixes #21756.

templates/zerver/help/desktop-app-install-guide.md

@@ -135,3 +135,4 @@ sudo apt install zulip
 
 * [Connect through a proxy](/help/connect-through-a-proxy)
 * [Use a custom certificate](/help/custom-certificates)
+* [View Zulip version](/help/view-zulip-version)

templates/zerver/help/include/sidebar_index.md

@@ -196,4 +196,5 @@
 * [View all bots in your organization](/help/view-all-bots-in-your-organization)
 
 ## Support
+* [View Zulip version](/help/view-zulip-version)
 * [Contact support](/help/contact-support)

templates/zerver/help/view-zulip-version.md

@@ -0,0 +1,49 @@
+# View Zulip version
+
+## Zulip server and web app version
+
+If your organization self-hosts Zulip, the server administrator will need to
+[upgrade Zulip][upgrade-zulip] in order for you to experience the latest
+improvements. You can check which [version][changelog] of the Zulip server and
+web app your organization is using.
+
+Zulip Cloud organizations are always updated to the latest version of Zulip.
+
+[upgrade-zulip]:
+    https://zulip.readthedocs.io/en/latest/production/upgrade-or-modify.html
+[changelog]: https://zulip.readthedocs.io/en/latest/overview/changelog.html
+
+### View Zulip server and web app version
+
+{start_tabs}
+
+{relative|gear|about-zulip}
+
+1. View the version number under **Zulip Server**.
+
+{end_tabs}
+
+!!! tip ""
+    If the Zulip server is running a custom branch, the **merge base**
+    will also be shown. It indicates which official version of Zulip
+    the custom branch is based on.
+
+## Zulip desktop app version
+
+The [Zulip desktop app](/apps/) has an independent version from the server and
+webapp.
+
+### View Zulip desktop app version
+
+{start_tabs}
+
+1. Click on the **Help** menu in the desktop app. The app version is shown in the
+   dropdown: **Zulip Desktop v...**
+
+{end_tabs}
+
+## Related articles
+
+* [Desktop installation guides](/help/desktop-app-install-guide)
+* [Upgrading Zulip][upgrade-zulip]
+* [Zulip release lifecycle](https://zulip.readthedocs.io/en/latest/overview/release-lifecycle.html)