Fix broken internal documentation links (#46945)
Several model and guide pages link to internal docs with malformed
relative paths (missing slash, wrong slug, stray './' or trailing '/'),
which resolve to non-existent pages. Repoint them to the correct
existing pages:
- ../quantization -> ../quantization/overview (5 occurrences)
- ./model_doc/gpt-oss -> ./model_doc/gpt_oss
- (SigLIP) -> (siglip)
- ./generation_strategies -> ../generation_strategies
- ../tasks/question-answering -> ../tasks/question_answering
- ../quantization/bitsandbytes/ -> ../quantization/bitsandbytes
- transformers_as_backend: ./ -> ../ for add_new_model, custom_models,
perf_infer_gpu_multi, contributing
