[docs] Add documentation for native SFT trainer in SkyRL

[SumanthRH]

While SFT is documented in the examples: https://github.com/NovaSky-AI/SkyRL/tree/0835f2145b2be486df4887b170ab980c117abe2a/examples/train/sft

It would be good to add documentation pages for the different features supported in the SFTTrainer :

Tutorials
  ....
SFT
  overview.md # Overview
  multi_dataset.md # Mixing Multiple Datasets 
  custom_sampler.md # Using Custom Samplers