Update sitemap.rst (#2631)

* Update sitemap.rst

* Update sitemap.rst

using the structure suggested by Kevin P instead

doc/sphinx/source/sitemap.rst

 :orphan:
-
+ 
 .. _sitemap.rst:
-
+ 
 .. toctree::
-   :maxdepth: 1 
-
-   Introduction <project/introduction.rst>
-   Framework support <frameworks/index.rst>
-   nGraph Core <core/overview.rst>
-   Backend support <backend-support/index.rst>
-   Distributed training <distr/index.rst>
-   Diagnostics and visualization <diagnostics/nbench.rst>
-   Tutorials <tutorials/index.rst>
-   Project <project/index.rst> 
-
+   :includehidden:
+ 
+   index.rst