help-relative: Add option for "Not subscribed" tab of channels overlay.

Updates the relative help link for "Browse and subscribe to channels"
in help center "Introduction to channels" article.

Fixes #31605.

help/introduction-to-channels.md

@@ -8,7 +8,7 @@
 
 {tab|desktop-web}
 
-{relative|channel|all}
+{relative|channel|not-subscribed}
 
 1. Scroll through the list of channels. You can use the **search box** near the
    top of the menu to filter the list by channel name or description.

zerver/lib/markdown/help_relative_links.py

@@ -117,6 +117,7 @@ def help_handle_match(key: str) -> str:
 
 channel_info = {
     "all": ["All channels", "/#channels/all"],
+    "not-subscribed": ["Not subscribed", "/#channels/notsubscribed"],
 }
 
 channel_all_instructions = """