docs: add Lighting pre-exported model loading to Tutorial 3

- Add two model types (SVDQ / Lighting) comparison table in download section
- Split Step 3 into Option A (SVDQ) and Option B (Lighting) with separate configs
- Clarify that Lighting pre-exported models load the same way as SVDQ,
  just with Lighting transformer weights and `lighting` backend
- Update noise FAQ to cover both backend types