[tlBMC/g3doc] Document Scheduler

Created documentation for the TaskScheduler module in `components/scheduler.md`.
Details its purpose, key components (TaskScheduler, Task), scheduling methods (`ScheduleAsync`, `RunAndScheduleAsync`, `ScheduleOneShotAsync`), and key features.
Based on `scheduler/scheduler.cc` and `scheduler/scheduler.h`.

PiperOrigin-RevId: 931382552
Change-Id: I5af79b181911c5fdca186084bacd8499e6b3c7d8