doc: canaan: Add K230 CanMV board

Add description of compiling u-boot for K230 CanMV.

Since the vendor's u-boot-spl verifies u-boot header [1], it is
necessary to use the Python script from vendor to add the header to the
u-boot image.

[1] https://github.com/kendryte/k230_sdk/blob/v1.8/src/little/uboot/board/canaan/common/k230_board_common.h#L52

Signed-off-by: Junhui Liu <junhui.liu@pigmoral.tech>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/doc/board/index.rst b/doc/board/index.rst
index b1c470e..74c4dd1 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -21,6 +21,7 @@
    beagle/index
    broadcom/index
    bsh/index
+   canaan/index
    cloos/index
    congatec/index
    coolpi/index