integrations: Update Asana integration doc to point to Zapier.

Updates the Asana documentation, which was a detailed version
of the Zapier documentation with screenshots specifically for
Asana, to instead start with the basic incoming webhook steps
and then point to the general Zapier documentation to complete
the integration.

This will be easier to maintain moving forward in the short
term as ideally we'll migrate to a system that documents all
of the integrations with Zulip that are available via Zapier.

Also, updates the current Zapier documentation to mention
Asana as one of the apps that can be integrated with Zulip.

templates/zerver/integrations/asana.md

@@ -1,60 +1,14 @@
 Get Zulip notifications for your Asana projects via Zapier!
 
-1.  {!create-stream.md!}
+1. {!create-stream.md!}
 
-1. {!create-bot-construct-url.md!}
+1. {!create-an-incoming-webhook.md!}
 
-1.  Start by setting up a [Zapier](https://zapier.com/) account.
-
-1.  Next, create a ZAP, picking Asana as the app you'd like
-    to receive notifications from as the **Trigger (Step 1)**:
-
-    ![Trigger](/static/images/integrations/asana/001.png)
-
-1.  Next, select the Asana event that you'd like to receive notifications
-    for (**Choose Trigger**), such as when you add a new **Task** in
-    an Asana project:
-
-    ![Trigger selection](/static/images/integrations/asana/002.png)
-
-1.  Next, click on **Connect a New Account** and follow the steps
-    to connect your Asana account to the Zap:
-
-    ![Account selection](/static/images/integrations/asana/003.png)
-
-1.  Select the Asana project you'd like to receive notifications for:
-
-    ![Project selection](/static/images/integrations/asana/004.png)
-
-1.  In **Action (Step 2)**, select **Webhooks by Zapier** as the app:
-
-    ![App selection](/static/images/integrations/asana/005.png)
-
-    and `POST` as the action:
-
-    ![Action selection](/static/images/integrations/asana/006.png)
-
-1.  Configure **Set up Webhooks by Zapier POST** as follows:
-
-     * `URL` is the URL we created above.
-     * `Payload Type` set to `JSON`.
-
-1.  Finally, configure **Data**. You have to add 2 fields:
-
-     * `subject` is the field corresponding to the topic of the message.
-     * `content` is the field corresponding to the content of the message.
-
-1.  You can format the content of the `content` and `subject` fields
-    in a number of ways as per your requirements.
-
-    Here's an example configuration:
-
-    ![Example configuration](/static/images/integrations/asana/007.png)
-
-{!congrats.md!}
-
-![Asana bot message](/static/images/integrations/asana/008.png)
+1. Continue with step 3 of the [Zapier documentation][1] to
+   finish integrating Zulip with Asana.
 
 You can repeat the above process and create Zaps for different projects
 and/or different kinds of Asana events that you'd like to be notified
 about in Zulip.
+
+[1]: ./zapier

zerver/webhooks/zapier/doc.md

@@ -14,7 +14,7 @@ sent by Zapier directly in Zulip.
 1. Click **Make a Zap!** in the upper left.
 
 1. Follow the instructions to select a Trigger App and Event (**When this happens ...** (Step 1)).
-   This could be an app like Trello, Gmail, Calendar, or anything else.
+   This could be an app like Asana, Trello, Gmail, Calendar, or anything else.
 
 1. **Do this ...** (Step 2): Search for Zulip, and click on the Zulip icon. Follow the instructions.