[Bugfix] Fix requirements paths in install instructions

[yingjun-mou]

Purpose

Updated all occurrences of wrong requirements/ paths to use the correct ones, assuming commands are run from the repository root.

The installation docs referenced requirements-build.txt and requirements-cpu.txt but the actual file are located at requirements/build.txt and requirements/cpu.txt. This caused pip install commands to fail if followed literally.

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
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• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

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[gemini-code-assist]

Code Review

This pull request aims to fix incorrect paths to requirements files. While the changes in the documentation file docs/getting_started/installation/cpu/s390x.inc.md are correct, the changes in docker/Dockerfile.ppc64le and docker/Dockerfile.s390x seem to introduce a build failure. These Dockerfiles build pyarrow from its source, and the requirements-build.txt file they refer to is from the arrow repository, not the vllm repository. The arrow repository does not have a requirements/ directory at the location where the command is run. I've left specific comments with suggestions to fix this.

[yingjun-mou]

Hi @hmellor, could you please review this PR? Thanks.

[hmellor]

LGTM! Thanks for fixing this