[AI-73] Add LangGraph integration documentation (#4442)

* Add LangGraph integration documentation

Add a full integration guide for the new LangGraph plugin
(temporalio/sdk-python#1448) covering both Graph API and Functional API
usage, Activity options, checkpointer setup, human-in-the-loop,
continue-as-new with caching, and links to samples.

Also adds LangGraph to the Python integrations index, sidebar,
plugins guide, and encyclopedia.

* Add Activity vs Workflow execution guidance for LangGraph

Add decision tree, primitive reference table, and detailed guidance
on when to use execute_in workflow vs the default Activity execution,
including constraints on conditional edge functions and interrupt().

* Fix broken link to workflow logic requirements

* Update LangGraph docs for required execute_in and removed features

The plugin now requires execute_in on every node/task with no Activity
default. Streaming via stream_writer and continue-as-new caching aren't
currently supported, so remove those sections. Add Runtime context and
Stores-not-supported sections from the plugin README, plus a warning
that LangGraph retry_policy raises an error.

* Point sample links to open PR branch and drop linter section

The samples-python langgraph_plugin samples are still on PR #289, not
main, so update the in-doc links to the PR branch. Drop the static
analysis section since the linter script was never committed.

* Point sample links to merged samples-python main branch

* Fix broken continue-as-new link

* Use bare domain for LangGraph SDK docs link text

* Tighten intro, drop decision tree and samples table

* Update docs/develop/python/integrations/langgraph.mdx

Co-authored-by: Brian Strauch <brian@brianstrauch.com>

* Address review: 3.11 streaming requirement, Functional API wording

- Add streaming from Workflow nodes to the Python 3.11 requirements list
- Reframe the Functional API blurb around orchestration rather than implying
  Graph API nodes can't use native Python control flow

---------

Co-authored-by: Brian Strauch <brian@brianstrauch.com>

Author: DABH

docs/develop/plugins-guide.mdx

@@ -42,6 +42,8 @@ If you prefer to learn by getting hands-on with code, check out some existing pl
 
 - Temporal's Python SDK ships with an
   [OpenAI Agents SDK](https://github.com/temporalio/sdk-python/tree/main/temporalio/contrib/openai_agents) plugin
+- Temporal's Python SDK ships with a
+  [LangGraph](https://github.com/temporalio/sdk-python/tree/main/temporalio/contrib/langgraph) plugin
 - [Temporal client and Worker plugin for Pydantic AI](https://github.com/pydantic/pydantic-ai/blob/757d40932ebb8ef00f25cc469ff44e9b267b1aa3/pydantic_ai_slim/pydantic_ai/durable_exec/temporal/__init__.py#L83)
 - Temporal's TypeScript SDK ships with an
   [OpenTelemetry Plugin](https://github.com/temporalio/sdk-typescript/blob/main/packages/interceptors-opentelemetry/src/plugin.ts)

docs/develop/python/integrations/index.mdx

@@ -24,6 +24,7 @@ The following integrations are available between the Temporal Python SDK and thi
 | ----------------- | --------------- | ------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------ |
 | Braintrust        | Observability   | [braintrust.dev](https://braintrust.dev/docs)                      | [Guide](./braintrust.mdx)                                                                              |
 | Google ADK        | Agent framework | [adk.dev](https://adk.dev/)                                        | [Guide](https://adk.dev/integrations/temporal/)                                                        |
+| LangGraph         | Agent framework | [langchain-ai.github.io](https://langchain-ai.github.io/langgraph/)            | [Guide](./langgraph.mdx)                                                                               |
 | LangSmith         | Observability   | [smith.langchain.com](https://docs.smith.langchain.com/)           | [Guide](./langsmith.mdx)                                                                               |
 | OpenAI Agents SDK | Agent framework | [openai.github.io](https://openai.github.io/openai-agents-python/) | [Guide](https://github.com/temporalio/sdk-python/blob/main/temporalio/contrib/openai_agents/README.md) |
 | Pydantic AI       | Agent framework | [ai.pydantic.dev](https://ai.pydantic.dev/)                        | [Guide](https://ai.pydantic.dev/durable_execution/temporal/)                                           |