[Doc]Add the user_guide doc file regarding fine-grained TP. (#5084)

### What this PR does / why we need it?
Add user guide for **Fine-Grained Tensor Parallelism** feature.  
Documents usage, supported components (`embedding`, `lm_head`, `o_proj`,
`mlp`/`dense_ffn`), model compatibility, and deployment guidelines.

- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

---------

Signed-off-by: zzhx1 <zzh_201018@outlook.com>
Signed-off-by: chenxiao <Jaychou1620@Gmail.com>
Signed-off-by: 秋刀鱼 <jaychou1620@Gmail.com>
Co-authored-by: chenxiao <Jaychou1620@Gmail.com>
Co-authored-by: Jade Zheng <zheng.shoujian@outlook.com>

docs/source/user_guide/feature_guide/index.md

@@ -18,5 +18,6 @@ kv_pool
 external_dp
 large_scale_ep
 ucm_deployment
+Fine_grained_TP
 speculative_decoding
 :::