docs: i2c: instantiating-devices: rearrange static instatiation

Among the "static" instantiation methods the "board file" method is
described first. Move it as last, since it is being replaced by the other
methods.

Also fix subsubsection heading syntax and remove the "Method 1[abc]"
prefix as the subsubsection structure clarifies the logical hierarchy.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>