docs: add guide for managing user context with Store

Add comprehensive documentation for managing user preferences using
LangGraph Store and static runtime context pattern.

- Demonstrate Store + Static Context integration
- Include practical examples for both Python and JavaScript
- Cover loading, updating, and dynamic field patterns
- Add best practices and related resources
- Update navigation in docs.json for both languages

Author: jjhpka

src/docs.json

@@ -179,7 +179,8 @@
                       "oss/python/langchain/human-in-the-loop",
                       "oss/python/langchain/multi-agent",
                       "oss/python/langchain/retrieval",
-                      "oss/python/langchain/long-term-memory"
+                      "oss/python/langchain/long-term-memory",
+                      "oss/python/langchain/manage-user-context"
                     ]
                   },
                   {
@@ -516,7 +517,8 @@
                       "oss/javascript/langchain/human-in-the-loop",
                       "oss/javascript/langchain/multi-agent",
                       "oss/javascript/langchain/retrieval",
-                      "oss/javascript/langchain/long-term-memory"
+                      "oss/javascript/langchain/long-term-memory",
+                      "oss/javascript/langchain/manage-user-context"
                     ]
                   },
                   {