### What this PR does / why we need it?
This PR adds description of preemption into FAQs in vLLM-Ascend. This
FAQ stats:
- how preemption affects the performance of a vLLM server.
- how reduce the negative impacts of preemption.
The reason why we add this FAQ is that we find that the origin
description of preemption in vLLM is not very straightforward. If
preemption causes performance drop, users might not be aware that this
is caused by Preemption.
### Does this PR introduce _any_ user-facing change?
No.
Signed-off-by: Angazenn <supperccell@163.com>
9e8da00f95