help-docs: Document "Copy link to stream" mobile app feature.

Adds a new tab with instructions for mobile app users.

Adds new macros with instructions for accessing the long-press menu.

Fixes: #22197.

templates/zerver/help/include/stream-long-press-menu-tip.md

@@ -0,0 +1,4 @@
+!!! tip ""
+
+    If you are viewing a single stream, you can access the long-press
+    menu from the bar at the top of the screen.

templates/zerver/help/include/stream-long-press-menu.md

@@ -0,0 +1 @@
+1. Press and hold a stream until the long-press menu appears.

templates/zerver/help/link-to-a-message-or-conversation.md

@@ -105,6 +105,14 @@ message](/help/quote-and-reply).
     If using Zulip in a browser, you can also click on a stream in the
     left sidebar, and copy the URL from your browser's address bar.
 
+{tab|mobile}
+
+{!stream-long-press-menu.md!}
+
+1. Tap **Copy link to stream**.
+
+{!stream-long-press-menu-tip.md!}
+
 {end_tabs}
 
 ## Related articles