doc: arch: Convert README.sandbox to reST

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

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/arch/index.rst b/doc/arch/index.rst
index d24662e..2980b77 100644
--- a/doc/arch/index.rst
+++ b/doc/arch/index.rst
@@ -12,5 +12,6 @@
    mips
    nds32
    nios2
+   sandbox
    sh
    x86