help: Document updated UI for subscribing/unsubscribing from streams.

Updates instructions for subscribing/unsubscribing to streams via
Stream settings on desktop/web, adding new SVG images for the icons.

Fixes #27753.

help/browse-and-subscribe-to-streams.md

@@ -9,8 +9,9 @@
 1. Scroll through the list of streams. You can use the **search box** near the
    top of the menu to filter the list by stream name or description.
 
-1. Hover over a stream, and click the gray checkmark to its left to subscribe to
-   the stream.
+1. Click the **plus**
+   (<img src="/static/images/help/desktop-web-plus-icon.svg" alt="plus" class="help-center-icon"/>)
+   icon to the left of a stream to subscribe to it.
 
 !!! tip ""
 

help/unsubscribe-from-a-stream.md

@@ -30,7 +30,9 @@ You can always unsubscribe from any stream in Zulip.
 
 {relative|stream|subscribed}
 
-1. Click the green checkmark to the left of a stream to unsubscribe from it.
+1. Click the **checkmark**
+   (<img src="/static/images/help/desktop-web-check-icon.svg" alt="checkmark" class="help-center-icon"/>)
+   icon to the left of a stream to unsubscribe from it.
 
 {tab|mobile}