[Docs] Explain difference between AIBrixOffloadingConnector vs AIBrixPDReuseConnector

### 🚀 Feature Description and Motivation

KV-centric P/D is not documented as one story. kvcache-offloading.rst covers offload, but there is no docs-site page explaining AIBrixOffloadingConnector vs AIBrixPDReuseConnector, shared store, PrisKV/InfiniStore backend choices, single-node no-RDMA path, or the comparison diagram requested in the PR.

### Use Case

give user clear guidance

### Proposed Solution

_No response_