docs: restructure to docs/, add guides and developer docs

- Rename assets/ to docs/, split into guides/ and developer/
- Add get-started.md: installation + 5-step quickstart
- Add guides/evaluation.md: 7 eval scripts with CLI args
- Add guides/distributed.md: DDP/FSDP, gradient accumulation, NCCL
- Add developer/internals.md: loss formulas, RoPE, KV cache math
- Add developer/cuda_kernels.md: build system, benchmarks, file layout
- Fix storage_format doc in preprocessing.md
- Update cross-references in README.md, README-zh-CN.md, Dockerfile

Dockerfile

@@ -43,7 +43,7 @@ ENV PATH="/opt/venv/bin:$PATH"
 # Copy application code
 COPY astrai/ ./astrai/
 COPY scripts/ ./scripts/
-COPY assets/ ./assets/
+COPY docs/ ./docs/
 COPY pyproject.toml .
 COPY README.md .