Add bitsandbytes (mock) and sentencepiece (real) to diffusers doc deps (#805)

The old diffusers-doc-builder container provided both; the light path
needs sentencepiece real (kolors' tokenizer imports it at module level)
and bitsandbytes mocked (quantization docs autodoc gated classes).
Requested in huggingface/diffusers#14134.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: mishig25

src/doc_builder/mock_deps/diffusers.txt

@@ -10,3 +10,5 @@ real:requests
 real:safetensors
 real:pillow
 real:transformers
+bitsandbytes
+real:sentencepiece  # kolors tokenizer imports it at module level