docs: add page on creating react agent from scratch (#1626)

---------

Co-authored-by: vbarda <vadym@langchain.dev>

docs/_scripts/copy_notebooks.py

@@ -72,6 +72,7 @@ _MANUAL = {
         "langsmith_graph_trace.png",
         "run-id-langsmith.ipynb",
         "react-agent-structured-output.ipynb",
+        "react-agent-from-scratch.ipynb"
     ],
     "tutorials": [
         "introduction.ipynb",

docs/docs/how-tos/index.md

@@ -98,3 +98,4 @@ Please note that here will we use a **prebuilt agent**. One of the big benefits
 - [How to add memory to a ReAct agent](create-react-agent-memory.ipynb)
 - [How to add a custom system prompt to a ReAct agent](create-react-agent-system-prompt.ipynb)
 - [How to add human-in-the-loop processes to a ReAct agent](create-react-agent-hitl.ipynb)
+- [How to create prebuilt ReAct agent from scratch](react-agent-from-scratch.ipynb)

docs/mkdocs.yml

@@ -183,6 +183,7 @@ nav:
           - Add memory to a ReAct agent: how-tos/create-react-agent-memory.ipynb
           - Add a system prompt to a ReAct agent: how-tos/create-react-agent-system-prompt.ipynb
           - Add Human-in-the-loop to a ReAct agent: how-tos/create-react-agent-hitl.ipynb
+          - Create prebuilt ReAct agent from scratch: how-tos/react-agent-from-scratch.ipynb
   - "Conceptual Guides":
       - "concepts/index.md"
       - LangGraph for Agentic Applications: concepts/high_level.md