doc: add missing docs for 'west zephyr-export'

Additional Zephyr extension commands

####################################

Aside from the :ref:`build, flash, and debug commands <west-build-flash-debug>`,

the zephyr tree extends the west command set with additional zephyr-specific

commands.

.. Add a per-page contents at the top of the page. This page is nested

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This page documents miscellaneous :ref:`west-zephyr-extensions`.

.. _west-boards:

Additional help about the formatting options can be found by running::

  west boards -h

.. _west-zephyr-export:

Installing CMake packages: ``west zephyr-export``

*************************************************

This command registers the current Zephyr installation as a CMake

config package in the CMake user package registry.

In Windows, the CMake user package registry is found in

``HKEY_CURRENT_USER\Software\Kitware\CMake\Packages``.

In Linux and MacOS, the CMake user package registry is found in.

:file:`~/.cmake/packages`.

You may run this command when setting up a Zephyr workspace. If you do,

application CMakeLists.txt files that are outside of your workspace will be

able to find the Zephyr repository with the following:

.. code-block:: cmake

   find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})

See :zephyr_file:`share/zephyr-package/cmake` for details.