docs: functional api (#3125)

* Concepts page for the functional API
* How-to guides that show functional API implementations
* API reference for entrypoint, task, entrypoint.final
* Add functional API version to the workflows

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Co-authored-by: Vadym Barda <vadym@langchain.dev>
Co-authored-by: ccurme <chester.curme@gmail.com>

docs/docs/tutorials/index.md

@@ -9,7 +9,7 @@ New to LangGraph or LLM app development? Read this material to get up and runnin
 ## Get Started 🚀 {#quick-start}
 
 - [LangGraph Quickstart](introduction.ipynb): Build a chatbot that can use tools and keep track of conversation history. Add human-in-the-loop capabilities and explore how time-travel works.
-- [LangGraph Cheatsheet For Common Workflows](workflows.ipynb): Overview of the most common workflows and agent architectures in LangGraph.
+- [Common Workflows](workflows/index.md): Overview of the most common workflows using LLMs implemented with LangGraph.
 - [LangGraph Server Quickstart](langgraph-platform/local-server.md): Launch a LangGraph server locally and interact with it using REST API and LangGraph Studio Web UI.
 - [LangGraph Template Quickstart](../concepts/template_applications.md): Start building with LangGraph Platform using a template application.
 - [Deploy with LangGraph Cloud Quickstart](../cloud/quick_start.md): Deploy a LangGraph app using LangGraph Cloud.