Docs/sound: Add documentation for userspace-driven ALSA timers

Add the documentation which describes the new userspace-driven timers
API introduced in this patch series. The documentation contains:

- Description of userspace-driven ALSA timers, what they are for
- Description of the timers API
- Example of how the timers can be created and triggered
- How the timers can be used as a timer sources for snd-aloop module

Suggested-by: Axel Holzinger <aholzinger@gmx.de>
Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20240813120701.171743-3-ivan.orlov0322@gmail.com