boards: st: nucleo_wb09ke: replace OpenOCD with pyOCD for debugging

pyOCD is installed along with Zephyr, and can debug apps on STM32WB09 once
the appropriate pack (already publicly available) is installed.

On the other hand, OpenOCD comes with the Zephyr SDK but only on Linux, and
both Zephyr's fork and upstream lack support for STM32WB09, which was added
by commit 89b053b660bcef803e19c89584de933158c59299 in ST's OpenOCD fork[1].
While pre-builts for this OpenOCD fork are provided by ST as part of other
software packages (e.g., STM32CubeIDE), it is less convenient to use and
the exact support script to use varies depending on which tool OpenOCD is
borrowed from.

Remove OpenOCD support for STM32WB09 entirely, replacing it with pyOCD, and
add instructions about pack installation in the board documentation.

While this also applies to flashing, STM32CubeProgrammer is the default
runner for this, and already supports STM32WB09 without any shenanigans
(i.e., a bare "west flash" will work with CubeProg) - as such, place the
notice only in the debugging section, which is the only operation CubeProg
does not support.

[1] https://github.com/STMicroelectronics/OpenOCD



Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>