feat(docs): comprehensive improvement (#3564)

* docs: comprehensive documentation improvements for humans and agents

New human docs:
- grpo.qmd: GRPO deep dive (async, rewards, IS correction, scaling)
- ebft.qmd: EBFT guide (structured/strided modes, feature extraction)
- choosing_method.qmd: decision tree for SFT vs LoRA vs DPO vs GRPO
- vllm_serving.qmd: vLLM setup for GRPO (server/colocate, LoRA sync)
- training_stability.qmd: monitoring, NaN debugging, OOM, healthy metrics

New agent docs:
- AGENTS_SFT.md: agent reference for supervised fine-tuning
- AGENTS_DPO.md: agent reference for preference learning (DPO/KTO/ORPO)

Updated existing docs:
- rlhf.qmd: cross-references to new GRPO/EBFT/choosing-method guides
- getting-started.qmd: reorganized Next Steps with links to new guides
- debugging.qmd: link to training stability guide
- _quarto.yml: added new pages to sidebar navigation

Removed:
- bak.agents.md: stale backup that confused agents

* docs: trim duplicated generic config from AGENTS_DPO.md

Remove boilerplate training params (optimizer, gradient_checkpointing,
flash_attention, etc.) from each method template. These are not
preference-learning-specific and are already covered in AGENTS_SFT.md.
Config templates now show only method-specific fields with a reference
to AGENTS_SFT.md for the rest.

* docs: deduplicate across new doc pages

- grpo.qmd: collapse vLLM setup section to brief config + link to
  vllm_serving.qmd; collapse IS correction to essentials + link;
  replace full monitoring tables with summary + link to
  training_stability.qmd
- vllm_serving.qmd: remove duplicated async/IS config reference tables
  (already in grpo.qmd config reference); replace full example config
  with link to grpo.qmd quick start
- ebft.qmd: trim generic training params in quick start config

* fix: train scripts

* feat: split files into cleaner parts

* fix: cleanup pretraining docs

---------

Co-authored-by: Wing Lian <wing.lian@gmail.com>

_quarto.yml

@@ -238,6 +238,7 @@ website:
         - section: "Getting Started"
           contents:
             - docs/getting-started.qmd
+            - docs/choosing_method.qmd
             - docs/installation.qmd
             - docs/inference.qmd
             - section: "Model Guides"
@@ -302,6 +303,9 @@ website:
           contents:
             - docs/multimodal.qmd
             - docs/rlhf.qmd
+            - docs/grpo.qmd
+            - docs/ebft.qmd
+            - docs/vllm_serving.qmd
             - docs/reward_modelling.qmd
             - docs/lr_groups.qmd
             - docs/lora_optims.qmd
@@ -334,6 +338,7 @@ website:
         - section: "Troubleshooting"
           contents:
             - docs/faq.qmd
+            - docs/training_stability.qmd
             - docs/debugging.qmd
             - docs/nccl.qmd