doc: board: Add missing board index.rst files

These board index.rst files are missing in previous html doc patch
series, hence it causes some build warnings. Add those files.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>