doc: arch: Convert README.NDS32 to reST

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.

This also merges README.N1213 contents into the new nds32.rst file.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/arch/index.rst b/doc/arch/index.rst
index 360b5d9..7f10df0 100644
--- a/doc/arch/index.rst
+++ b/doc/arch/index.rst
@@ -8,4 +8,5 @@
 
    arm64
    mips
+   nds32
    x86