System Message: ERROR/3 (docs/release/installation/index.rst, line 9)

Unknown directive type "toctree".

.. toctree::
   :numbered:
   :maxdepth: 4

   abstract.rst
   introduction.rst
   architecture.rst
   requirements.rst
   baremetal.rst
   virtual.rst
   upstream.rst
   verification.rst
   troubleshooting.rst
   references.rst