doc: xen: Add Xen guest ARM64 board documentation

Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>
diff --git a/doc/board/index.rst b/doc/board/index.rst
index 0a15899..63935ab 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -22,4 +22,5 @@
    st/index
    tbs/index
    toradex/index
+   xen/index
    xilinx/index