doc: board: Add minimal documentation for Sielaff i.MX6 Solo board

Describe how to build and boot for the Sielaff i.MX6 Solo board.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
diff --git a/doc/board/index.rst b/doc/board/index.rst
index d0f9f35..62357c9 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -42,6 +42,7 @@
    renesas/index
    rockchip/index
    samsung/index
+   sielaff/index
    siemens/index
    sifive/index
    sipeed/index