[docs] LangGraph / LangGraph Platform docs updates (#4479)

Main changes made:
- Add top level horizontal tabs
- Reorganize the sidenav
- Build out README/index page
- Consolidate how-tos under each section
- Remove duplicate content

---------

Signed-off-by: William Fu-Hinthorn <13333726+hinthornw@users.noreply.github.com>
Co-authored-by: Tat Dat Duong <david@duong.cz>
Co-authored-by: Sydney Runkle <sydneymarierunkle@gmail.com>
Co-authored-by: William Fu-Hinthorn <13333726+hinthornw@users.noreply.github.com>
Co-authored-by: Vadym Barda <vadym@langchain.dev>
Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>
Co-authored-by: ccurme <chester.curme@gmail.com>
Co-authored-by: Andrew Nguonly <andrewnguonly@users.noreply.github.com>
Co-authored-by: David Asamu <david.asamu@langchain.dev>
Co-authored-by: infra <mukil@langchain.dev>
Co-authored-by: Arjun Natarajan <arjun@langchain.dev>

docs/docs/tutorials/auth/getting_started.md

@@ -11,7 +11,7 @@
     This guide assumes basic familiarity with the following concepts:
 
       *  [**Authentication & Access Control**](../../concepts/auth.md)
-      *  [**LangGraph Platform**](../../concepts/index.md#langgraph-platform)
+      *  [**LangGraph Platform**](../../concepts/langgraph_platform.md)
 
 !!! note "Python only"