[Docs]: Document supported SIST platforms and build requirements

[harryswift01]

Documentation Update

Add a clear guide explaining which platforms SIST supports and what is required to build or run it.

Why Is This Needed?

The repository contains binaries for several environments, but their support status and build requirements are unclear.

This information is also needed before deciding which Conda packages can be published.

Suggested Changes

Document:

• Supported operating systems and architectures.
• x86, x86-64, and ppc64 support.
• QEMU and container-based environments.
• Compiler and runtime-library requirements.
• Quad-precision restrictions.
• Existing pre-built binaries.

Clearly distinguish tested, experimental, and unsupported platforms.

Done When

• Supported and tested platforms are listed.
• Compiler and runtime requirements are documented.
• Existing binaries are linked to their known environments.
• Platform-specific limitations are clear.
• Users can check whether their system is suitable before installation.