docs(guide): add installation without Docker guide

[dafahaha]

What

Add a comprehensive step-by-step guide for installing Relax without Docker, derived from the official docker/Dockerfile. The guide covers:

1. Prerequisites (hardware, software, environment variables)
2. System dependencies
3. PyTorch 2.9.0 with CUDA 12.9
4. Core CUDA extensions (flash-attn v2/v3, transformer-engine, torch_memory_saver, apex)
5. Megatron-LM via Megatron-Bridge (with custom patch)
6. SGLang with custom R3 patch + patched sglang-router
7. TransferQueue
8. Python dependencies and Relax installation
9. INT4 QAT CUDA extension
10. Environment variables summary
11. Verification steps
12. Troubleshooting (compilation errors, import errors, NCCL issues, patch conflicts)

Both English and Chinese versions are provided.

Why

Closes #119. Users in restricted environments (shared servers, HPC without container runtime) cannot use Docker. The current "Install from Source" section in installation.md is too brief — pip install -r requirements.txt && pip install -e . does not install the core CUDA dependencies (Megatron-LM, SGLang, flash-attn, etc.) because pyproject.toml has no [project.dependencies].

How has this been tested?

• Documentation only change. Content is derived from docker/Dockerfile (the authoritative build reference).
• Both English and Chinese versions follow the same structure.
• Conservative wording: explicitly states "derived from Dockerfile", "if you encounter compilation errors, refer to the Dockerfile as the authoritative source", and "passing import checks does not guarantee full training will work".

Note

Sidebar navigation links in .vitepress/config.mts and cross-references in existing installation.md are not included in this PR to keep the change focused. They can be added in a follow-up PR. The exact lines to add:

English sidebar (after Installation):
{ text: 'Installation without Docker', link: '/en/guide/installation-without-docker' },

Chinese sidebar (after 安装):
{ text: '无 Docker 环境安装', link: '/zh/guide/installation-without-docker' },

Checklist

• I have read the CONTRIBUTING.md document
• My code follows the code style of this project (Ruff, line-length 119)
• I have updated the documentation accordingly
• Both English and Chinese versions are provided
• The PR title follows Conventional Commits format
• Commits are signed off