help: Fix instances of empty url.

The linting in our new help center does not allow empty urls, so we
need to fix it at source before conversion. We replace these with
destinations on the current page.

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

@@ -7,7 +7,7 @@ tools.
 
 ## Link to a channel within Zulip
 
-Channel links are automatically formatted as [#channel name]().
+Channel links are automatically formatted as [#channel name](#link-to-a-channel-within-zulip).
 
 {start_tabs}
 
@@ -35,7 +35,7 @@ You can create a channel link manually by typing `#**channel name**`
 
 ## Link to a topic within Zulip
 
-Topic links are automatically formatted as [#channel > topic]().
+Topic links are automatically formatted as [#channel > topic](#link-to-a-topic-within-zulip).
 
 {start_tabs}
 

help/support-zulip-project.md

@@ -55,7 +55,7 @@ Collective](https://opencollective.com/zulip).
   your experience with Zulip (good or bad) helps them evaluate whether Zulip
   might work for their needs.
 
-- **Subscribe** to [our blog](https://blog.zulip.org/), and share our posts.
+* **Subscribe** to [our blog](https://blog.zulip.org/), and share our posts.
 
 * **Mention** Zulip on social media, or like and retweet [Zulip's
   tweets](https://twitter.com/zulip), or retoot [Zulip's

help/using-zulip-for-a-class.md

@@ -30,7 +30,7 @@ Share lecture notes and reading materials with [drag-and-drop file
 uploads](/help/share-and-upload-files), for example:
 
 * **\#Unit 3: Sorting algorithms** > **lecture notes**: Here are the notes
-from today’s lecture. [lecture notes 10/2.pdf]() You can view a recording of the lecture [here]().
+from today’s lecture. [lecture notes 10/2.pdf](/help/using-zulip-for-a-class) You can view a recording of the lecture [here](/help/using-zulip-for-a-class).
 
 ### Formatting tips