hrtimer: add usage examples to documentation

Add documentation examples showing various ways to use hrtimers:

- Box-allocated timers with shared state in Arc.
- Arc-allocated timers.
- Stack-based timers for scoped usage.
- Mutable stack-based timers with shared state.

Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://msgid.link/20260219-hrtimer-examples-v6-19-rc1-v2-1-810cc06ca9f6@kernel.org
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>