chore: import upstream snapshot with attribution

site/docs/providers/webhook.md

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+---
+sidebar_label: Generic Webhook
+description: Configure webhook integrations to trigger custom LLM flows and prompt chains with HTTP POST requests, enabling seamless API-based testing and evaluation
+---
+
+# Generic Webhook
+
+The webhook provider can be useful for triggering more complex flows or prompt chains end to end in your app.
+
+It is specified like so:
+
+```yaml
+providers:
+  - webhook:http://example.com/webhook
+```
+
+promptfoo will send an HTTP POST request with the following JSON payload:
+
+```json
+{
+  "prompt": "..."
+}
+```
+
+It expects a JSON response in this format:
+
+```json
+{
+  "output": "..."
+}
+```
+
+## Passing custom properties
+
+It is possible to set webhook provider properties under the `config` key by using a more verbose format:
+
+```yaml
+providers:
+  - id: webhook:http://example.com/webhook
+    config:
+      foo: bar
+      test: 123
+```
+
+These config properties will be passed through in the JSON request payload:
+
+```json
+{
+  "prompt": "...",
+  "config": {
+    "foo": "bar",
+    "test": 123
+  }
+}
+```