boards: st: stm32g0316: Use openocd as debug interface.

Default pyocd configuration doesn't allow debugging on this board.
Now that Zephyr openocd version support stm32g0 series, use it instead
of pyocd.
This commit provides working openocd configuration for this board
an update documentation.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>