doc: xen: Add Xen guest ARM64 board documentation

Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>
diff --git a/doc/board/xen/index.rst b/doc/board/xen/index.rst
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+.. SPDX-License-Identifier: GPL-2.0+
+
+XenGuestARM64
+=============
+
+.. toctree::
+   :maxdepth: 2
+
+   xenguest_arm64