docs: Add trigger and connection pages of webhooks app

packages/docs/pages/apps/webhooks/connection.md

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+# Webhooks
+
+Webhooks is a built-in app shipped with Automatisch, and it doesn't need to authenticate with any other external service to run.
+
+## How to use
+
+You will be given a webhook URL in the test substep on the editor page, and you can use it to send a GET, POST, PUT, or PATCH request to Automatisch to trigger the flow.

packages/docs/pages/apps/webhooks/triggers.md

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+---
+favicon: /favicons/webhooks.svg
+items:
+  - name: Catch raw webhook
+    desc: Triggers when the webhook receives a request.
+---
+
+<script setup>
+  import CustomListing from '../../components/CustomListing.vue'
+</script>
+
+<CustomListing />