### What this PR does / why we need it?
Add user guide of speculative decoding that includes n-grams, EAGLE,
MTP, and suffix.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c
Signed-off-by: zhaomingyu <zhaomingyu13@h-partners.com>