docs: Add documentation for s390x cpu implementation (#14198)

Signed-off-by: Dilip Gowda Bhagavan <dilip.bhagavan@ibm.com>
Co-authored-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/source/getting_started/installation/cpu/s390x.inc.md

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+# Installation
+
+vLLM has experimental support for s390x architecture on IBM Z platform. For now, users shall build from the vLLM source to natively run on IBM Z platform.
+
+Currently the CPU implementation for s390x architecture supports FP32 datatype only.
+
+:::{attention}
+There are no pre-built wheels or images for this device, so you must build vLLM from source.
+:::
+
+## Requirements
+
+- OS: `Linux`
+- SDK: `gcc/g++ >= 12.3.0` or later with Command Line Tools
+- Instruction Set Architecture (ISA): VXE support is required. Works with Z14 and above.
+- Build install python packages: `pyarrow`, `torch` and `torchvision`
+
+## Set up using Python
+
+### Pre-built wheels
+
+### Build wheel from source
+
+Install the following packages from the package manager before building the vLLM. For example on RHEL 9.4:
+
+```console
+dnf install -y \
+    which procps findutils tar vim git gcc g++ make patch make cython zlib-devel \
+    libjpeg-turbo-devel libtiff-devel libpng-devel libwebp-devel freetype-devel harfbuzz-devel \
+    openssl-devel openblas openblas-devel wget autoconf automake libtool cmake numactl-devel
+```
+
+Install rust>=1.80 which is needed for `outlines-core` and `uvloop` python packages installation.
+
+```console
+curl https://sh.rustup.rs -sSf | sh -s -- -y && \
+    . "$HOME/.cargo/env"
+```
+
+Execute the following commands to build and install vLLM from the source.
+
+::::{tip}
+Please build the following dependencies, `torchvision`, `pyarrow` from the source before building vLLM.
+::::
+
+```console
+    sed -i '/^torch/d' requirements-build.txt    # remove torch from requirements-build.txt since we use nightly builds
+    pip install -v \
+        --extra-index-url https://download.pytorch.org/whl/nightly/cpu \
+        -r requirements-build.txt \
+        -r requirements-cpu.txt \
+    VLLM_TARGET_DEVICE=cpu python setup.py bdist_wheel && \
+    pip install dist/*.whl
+```
+
+## Set up using Docker
+
+### Pre-built images
+
+### Build image from source
+
+## Extra information