docs: expand LoRA training and serving guide

[yushengsu-thu]

Summary

• rewrite the LoRA guide around the actual training-to-serving lifecycle
• distinguish the Megatron-Bridge and native/raw LoRA implementation paths, including current model coverage and the native-first maintenance direction
• document core arguments, MoE layouts, checkpoint behavior, colocated and remote synchronization, and current limitations
• add historical GLM-5.2 BF16/FP8 validation evidence and two locally stored FP8 experiment figures while keeping configuration-specific results clearly scoped
• document the draft single-LoRA agentic recipe and its current limitations
• describe the dataset-driven multi-LoRA implementation on main and the future Tinker-compatible operation/backend work in [multi-lora] (WIP) 1/n tinker-backend: tinker-compatible operation backend for multi-LoRA #2273 and [multi-lora] 2/n tinker-frontend: SDK-compatible REST frontend (tinker==0.24.1 JSON subset) #2346

Why

The existing page had drifted from the implementation. In particular, it described LoRA as colocate-only, treated low-precision training support too broadly, omitted multi-node broadcast and multi-LoRA, and did not distinguish the Bridge, native, agentic, and Tinker roadmap boundaries. This update aligns the guide with current code, tests, launchers, and open roadmap proposals.

Status boundaries called out in the doc

• current main has general Bridge LoRA plus Inkling-specific native LoRA; generalized native coverage remains in open [lora] Native LoRA for standard Megatron-core raw mode models #1792
• current multi-LoRA is Bridge-based and dataset-driven; native multi-LoRA remains tracked in [RFC] lora to-do #2141
• the Tinker operation backend ([multi-lora] (WIP) 1/n tinker-backend: tinker-compatible operation backend for multi-LoRA #2273) and SDK-compatible REST frontend ([multi-lora] 2/n tinker-frontend: SDK-compatible REST frontend (tinker==0.24.1 JSON subset) #2346) are open roadmap work, not released features

Validation

• git diff origin/main...HEAD --check
• verified all local /assets/images/... references resolve to committed PNG files
• verified Markdown fences and Mintlify Note / Warning components are balanced
• independently cross-checked the model matrix, metrics, and current-vs-roadmap claims against code, tests, launchers, and open roadmap PRs