help: Rename "Starting a new private thread" to "... new direct message".

With the private messages -> direct messages migration, we should
rename the "Starting a new private thread" help center article.

- Renames article to "Starting a new direct message"
- Updates relevant section in /help/getting-started-with-zulip
- Fixes typo in /help/send-group-dm
- Updates file names and adds URL redirect.

Fixes #25506.

help/getting-started-with-zulip.md

@@ -48,9 +48,9 @@ Like your email inbox, Zulip works best if you read it topic-by-topic.
 
 {!how-to-start-a-new-topic.md!}
 
-### Starting a new private thread
+### Starting a new direct message
 
-{!starting-a-new-private-thread.md!}
+{!starting-a-new-direct-message.md!}
 
 ### Responding to an existing thread
 

help/include/send-group-dm.md

@@ -13,7 +13,7 @@
 
 !!! tip ""
 
-    You can switch from composing a stream message to a composing a direct
+    You can switch from composing a stream message to composing a direct
     message by selecting **Direct message** from the dropdown in the upper left
     of the compose box.
 

help/include/sidebar_index.md

@@ -14,7 +14,7 @@
 * [Reading topics](/help/reading-topics)
 * [Reading direct messages (DMs)](/help/reading-dms)
 * [Starting a new topic](/help/starting-a-new-topic)
-* [Starting a new private thread](/help/starting-a-new-private-thread)
+* [Starting a new direct message](/help/starting-a-new-direct-message)
 * [Replying to messages](/help/replying-to-messages)
 * [Messaging tips & tricks](/help/messaging-tips)
 * [Keyboard shortcuts](/help/keyboard-shortcuts)

help/include/starting-a-new-direct-message.md

@@ -2,5 +2,5 @@
 
 !!! tip ""
 
-    Rather than kicking off a group thread, consider starting the
+    Rather than kicking off a group direct message, consider starting the
     conversation in a new topic to make it easier to browse later on.

help/starting-a-new-direct-message.md

@@ -1,6 +1,6 @@
-# Starting a new private thread
+# Starting a new direct message
 
-{!starting-a-new-private-thread.md!}
+{!starting-a-new-direct-message.md!}
 
 ## Related articles
 

zerver/lib/url_redirects.py

@@ -74,6 +74,7 @@ HELP_DOCUMENTATION_REDIRECTS: List[URLRedirect] = [
     URLRedirect("/help/night-mode", "/help/dark-theme"),
     URLRedirect("/help/enable-emoticon-translations", "/help/configure-emoticon-translations"),
     URLRedirect("/help/web-public-streams", "/help/public-access-option"),
+    URLRedirect("/help/starting-a-new-private-thread", "/help/starting-a-new-direct-message"),
 ]
 
 LANDING_PAGE_REDIRECTS = [