doc: board: Add Google Chromebook Samus board doc

This extracts Google Chromebook Samus 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/google/index.rst b/doc/board/google/index.rst
index 93833c5..7f557fe 100644
--- a/doc/board/google/index.rst
+++ b/doc/board/google/index.rst
@@ -7,3 +7,4 @@
    :maxdepth: 2
 
    chromebook_link
+   chromebook_samus