webhooks: Recommend creating "incoming webhook" bot.

In all our webhooks' docs, we now recommend creating a bot with the
"Incoming webhook" bot type.

templates/zerver/help/include/create-bot-construct-url.md

@@ -1,6 +1,17 @@
-Next, on your {{ settings_html|safe }}, create a bot for
-{{ integration_display_name }}. Construct the URL for the
-{{ integration_display_name }} bot using the bot API key and stream name:
+Next, on your {{ settings_html|safe }},
+[create a bot](/help/add-a-bot-or-integration) for
+{{ integration_display_name }}. Make sure that you select
+**Incoming webhook** as the **Bot type**:
+
+![](/static/images/help/bot_types.png)
+
+The API keys for "Incoming webhook" bots are limited to only
+sending messages via webhooks. Thus, this bot type lessens
+the security risks associated with exposing the bot's API
+key to third-party services.
+
+Construct the URL for the {{ integration_display_name }}
+bot using the bot API key and stream name:
 
 {!webhook-url.md!}