doc: merge coding_style -> conventions.rst

This removes warning about duplicate symbol (of _coding_style, due to
the inclusion) plus also a very short file which can be very well
inside conventions.rst.

Change-Id: I7b8467a0a845225a4fe4356f012f60ab0ea202aa
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>