Documentation pps.rst: add PPS generators documentation

This patch adds some examples about how to register a new PPS
generator in the system, and how to manage it.

Signed-off-by: Rodolfo Giometti <giometti@enneenne.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20241108073115.759039-4-giometti@enneenne.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>