doc: board: Add Intel Crown Bay board doc

This extracts Intel Crown Bay board specific information from
README.x86, converts plain text documentation to reST format 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 b47f672..99b5d2f 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -5,3 +5,5 @@
 
 .. toctree::
    :maxdepth: 2
+
+   intel/index