Add trainers taxonomy to docs (#4195)

Author: sergiopaniego

docs/source/index.md

@@ -7,6 +7,44 @@
 TRL is a full stack library where we provide a set of tools to train transformer language models with methods like Supervised Fine-Tuning (SFT), Group Relative Policy Optimization (GRPO), Direct Preference Optimization (DPO), Reward Modeling, and more.
 The library is integrated with 🤗 [transformers](https://github.com/huggingface/transformers).
 
+Below is the current list of TRL trainers, organized by method type (⚡️ = vLLM support).
+
+<div style="display: flex; justify-content: space-between; width: 100%; gap: 2rem;">
+
+<div style="flex: 1; min-width: 0;">
+
+**Online methods**
+- [`GRPOTrainer`] ⚡️
+- [`RLOOTrainer`] ⚡️
+- [`OnlineDPOTrainer`] ⚡️
+- [`NashMDTrainer`] ⚡️
+- [`XPOTrainer`] ⚡️
+- [`PPOTrainer`]
+
+**Reward modeling**
+- [`PRMTrainer`]
+- [`RewardTrainer`]
+
+</div>
+
+<div style="flex: 1; min-width: 0;">
+
+**Offline methods**
+- [`SFTTrainer`]
+- [`DPOTrainer`]
+- [`ORPOTrainer`]
+- [`BCOTrainer`]
+- [`CPOTrainer`]
+- [`KTOTrainer`]
+
+**Knowledge distillation**
+- [`GKDTrainer`]
+
+</div>
+
+</div>
+
+
 ## 🎉 What's New
 
 **✨ OpenAI GPT OSS Support**: TRL now fully supports fine-tuning the latest [OpenAI GPT OSS models](https://huggingface.co/collections/openai/gpt-oss-68911959590a1634ba11c7a4)! Check out the: