[Doc] Update README and installation docs for v0.26.0 (#5715)

Signed-off-by: hsliuustc0106 <liuhongsheng4@huawei.com>

docs/getting_started/installation/README.md

@@ -1,5 +1,8 @@
 # Installation
 
+!!! important
+    vLLM-Omni is released against the matching upstream vLLM major/minor version. vLLM-Omni 0.26.x requires vLLM 0.26.x; the stable v0.26.0 instructions pin vLLM 0.26.0.
+
 vLLM-Omni supports the following hardware platforms:
 
 - [GPU](gpu.md)

docs/getting_started/installation/gpu/cuda.inc.md

@@ -5,7 +5,7 @@
 # --8<-- [end:requirements]
 # --8<-- [start:set-up-using-python]
 
-vLLM-Omni depends vLLM. So please follow instructions below mainly for vLLM.
+vLLM-Omni depends on the matching major/minor release of vLLM. The vLLM-Omni 0.26.x release line uses vLLM 0.26.x.
 
 !!! note
     PyTorch installed via `conda` will statically link `NCCL` library, which can cause issues when vLLM tries to use `NCCL`. See <gh-issue:8420> for more details.
@@ -124,7 +124,7 @@ If you want to specify the base vLLM version:
 ```bash
 DOCKER_BUILDKIT=1 docker build \
   -f docker/Dockerfile.cuda \
-  --build-arg BASE_IMAGE=vllm/vllm-openai:v0.22.1 \
+  --build-arg BASE_IMAGE=vllm/vllm-openai:v0.26.0 \
   -t vllm-omni-cuda .
 ```
 

docs/getting_started/installation/npu/npu.inc.md

@@ -53,16 +53,20 @@ We are keeping [issue #886](https://github.com/vllm-project/vllm-omni/issues/886
 You can also build vLLM-Omni from the latest main branch if you want to use the latest features or bug fixes. (But sometimes it will break for a while. You can check [issue #886](https://github.com/vllm-project/vllm-omni/issues/886) for the status of the latest commit of vLLM-Omni main branch on NPU.)
 
 ```bash
-# Pin vLLM version to 0.18.0
-git clone -b v0.18.0 https://github.com/vllm-project/vllm.git
+# Pin vLLM and vLLM-Ascend to the v0.26 release line
+git clone -b v0.26.0 https://github.com/vllm-project/vllm.git
+cd vllm
 VLLM_TARGET_DEVICE=empty pip install -v -e .
+cd ..
 
-git clone -b v0.18.0rc1 https://github.com/vllm-project/vllm-ascend.git
+git clone -b releases/v0.26.0rc https://github.com/vllm-project/vllm-ascend.git
+cd vllm-ascend
 pip install -v -e .
+cd ..
 
 # Install vLLM-Omni from the latest main branch
 git clone https://github.com/vllm-project/vllm-omni.git
-cd /vllm-workspace/vllm-omni
+cd vllm-omni
 pip install -v -e . --no-build-isolation
 # or VLLM_OMNI_TARGET_DEVICE=npu pip install -v -e .
 export VLLM_WORKER_MULTIPROC_METHOD=spawn