docs: Add documentation for s390x cpu implementation

[dilipgb]

This PR provides documentation for building vLLM cpu implementation for s390x architecture. PR for s390x cpu implementation is approved and awaiting merge. #12613

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[dilipgb]

@dtrifiro can you please take a look at this PR.

[hmellor]

From a docs perspective, this looks good to me.

Only suggestion is to make sure torchvision doesn't have a space.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @dilipgb.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[dilipgb]

@hmellor, I have rebased the PR and updated installation.md with path changes due to rebase. Also addressed whitespace in torchvision. Can we merge the PR, if this looks good?