[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>

.github/workflows/deploy_docs.yml

@@ -38,12 +38,13 @@ jobs:
         with:
           filter: "docs/docs/**"
 
-  run-changed-notebooks:
-    needs: get-changed-files
-    uses: ./.github/workflows/run_notebooks.yml
-    secrets: inherit
-    with:
-      changed-files: ${{ needs.get-changed-files.outputs.changed-files }}
+  # TODO: Uncomment this to run on PRs
+  # run-changed-notebooks:
+  #   needs: get-changed-files
+  #   uses: ./.github/workflows/run_notebooks.yml
+  #   secrets: inherit
+  #   with:
+  #     changed-files: ${{ needs.get-changed-files.outputs.changed-files }}
 
   deploy:
     # needs: run-changed-notebooks
@@ -98,6 +99,7 @@ jobs:
       - name: Check links in notebooks
         env:
           LANGCHAIN_API_KEY: test
+        if: github.event_name == 'schedule'
         run: |
           if [ "${{ github.event_name }}" == "schedule" ]; then
             echo "Running link check on all HTML files matching notebooks in docs directory..."