Add document for parallel computation.

[yfukai]

For the parallel_backend option in

laptrack/src/laptrack/_tracking.py

Lines 388 to 393 in 30c1afc

	parallel_backend: ParallelBackend = Field(
	"serial",
	description="The parallelization strategy. "
	+ f"Must be one of {', '.join([ps.name for ps in ParallelBackend])}.",
	exclude=True,
	)

we need some documentation