doc: riscv: Add documentation for Sipeed Maix Bit

This patch adds documentation for the Sipeed Maix bit, and more generally
for the Kendryte K210 processor.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
diff --git a/doc/board/index.rst b/doc/board/index.rst
index 19733a6..0a15899 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -18,6 +18,7 @@
    renesas/index
    rockchip/index
    sifive/index
+   sipeed/index
    st/index
    tbs/index
    toradex/index