doc: sphinx: Add sphinx-prompt

Sphinx-prompt[1] helps bring-in '.. prompt::' option that allows a
better rendered documentation, yet be able to copy paste without
picking up the prompt from rendered documentation.

[1] https://lore.kernel.org/all/87fs48rgto.fsf@baylibre.com/


Suggested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>