doc: board: Add AndesTech ax25-ae350 board doc

This converts README.AX25 and README.ae350 plain text documentation
to reST format, merges them into one ax25-ae350 doc, and adds it to
Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/board/index.rst b/doc/board/index.rst
index 6d2356f..1cf0bc6 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -6,6 +6,7 @@
 .. toctree::
    :maxdepth: 2
 
+   AndesTech/index
    coreboot/index
    emulation/index
    google/index