[Doc] Add user guide of speculative decoding (#5074)

### 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>

docs/source/user_guide/feature_guide/index.md

@@ -18,4 +18,5 @@ kv_pool
 external_dp
 large_scale_ep
 ucm_deployment
+speculative_decoding
 :::