Document invocable carrier SDK reference

Add the invocable carrier reference page and navigation entry for the Python SDK docs.

Author: durable-workflow-ops

docs/index.md

@@ -25,6 +25,7 @@ pip install 'durable-workflow[prometheus]'
 - **[Metrics](reference/metrics.md)** — pluggable recorders, including a Prometheus adapter.
 - **[Serializer](reference/serializer.md)** — payload encoding and decoding helpers.
 - **[External storage](reference/external_storage.md)** — reference envelopes and driver contracts for large-payload offload.
+- **[Invocable carriers](reference/invocable.md)** — activity-grade external-task adapter for HTTP and serverless handlers.
 - **[Sync helpers](reference/sync.md)** — blocking wrappers around the async client for scripts and tests.
 
 ## Versioning

docs/reference/invocable.md

@@ -0,0 +1,23 @@
+# Invocable Carriers
+
+`InvocableActivityHandler` is the reference adapter for activity-grade external
+execution. It consumes the stable external-task input envelope, rejects
+workflow-task inputs, dispatches a registered activity handler, and returns the
+stable external-task result envelope.
+
+Use it for HTTP, serverless, and other bounded carriers that should execute
+activities without becoming workflow runtimes. Operators and agents can inspect
+the same result contract as poll-based workers: success, retryability,
+application failures, cancellations, decode failures, unsupported payloads, and
+deadline-exceeded failures all stay in the structured result envelope.
+
+```python
+from durable_workflow import InvocableActivityHandler
+
+
+async def handle_request(request_json: dict) -> dict:
+    adapter = InvocableActivityHandler({"billing.charge-card": charge_card})
+    return await adapter.handle(request_json)
+```
+
+::: durable_workflow.invocable

mkdocs.yml

@@ -74,6 +74,7 @@ nav:
       - External storage: reference/external_storage.md
       - External task input: reference/external_task_input.md
       - External task result: reference/external_task_result.md
+      - Invocable carriers: reference/invocable.md
       - Metrics: reference/metrics.md
       - Sync helpers: reference/sync.md
       - Testing: reference/testing.md