docs: add s390x build documentation

[taronaeo]

This PR includes the s390x specific build documentation. I've decided to separate it from the default build.md because we're a big-endian architecture that need more documentation to get up and running.

Feel free to let me know if there are other ways to approach this documentation.

Please review this pull request and consider merging into the main repository. Thank you!

[Copilot]

Pull Request Overview

This PR adds dedicated build documentation for the s390x (IBM Z & LinuxONE) architecture, including CPU build instructions, model conversion steps, and performance tuning tips.

• Introduces docs/build-s390x.md with s390x-specific build and conversion guidance
• Covers BLAS-enabled builds, GGUF model endian conversions, and IBM accelerator notes
• Details architectural performance tuning and support channels

Comments suppressed due to low confidence (4)

docs/build-s390x.md:2

• The link to build.md may not resolve correctly; prefix it with a relative path (e.g., ./build.md) or use the full repository path to ensure it works from within docs/.

> This build documentation is specific only to IBM Z & LinuxONE mainframes (s390x). You can find the build documentation for other architectures: [build.md](build.md).

docs/build-s390x.md:73

• The hyperlink hf.co/collections/... is missing the https:// prefix and will render as a relative link; update it to https://hf.co/collections/....

    You can find popular models pre-converted and verified at [s390x Ready Models](hf.co/collections/taronaeo/s390x-ready-models-672765393af438d0ccb72a08).

docs/build-s390x.md:120

• [nitpick] The phrase “No direction at the moment” is vague; consider rephrasing to something like “No support currently available” for clarity.

*Only available in IBM z16 or later system. No direction at the moment.*

docs/build-s390x.md:124

• [nitpick] The wording here is unclear; consider changing to “Not supported at this time” or similar to make it explicit.

*No direction at the moment.*