[Docs] Create quickstart guide

[Abdennacer-Badaoui]

Add quickstart documentation with basic usage examples for:

• 8-bit/4-bit quantization
• QLoRA fine-tuning
• 8-bit optimizers

[TimDettmers]

PR Review: #1872 — [Docs] Create quickstart guide

Replaces the placeholder quickstart page with a comprehensive guide covering installation, 8-bit/4-bit inference, QLoRA fine-tuning, 8-bit optimizers, and custom quantized layers. A welcome contribution that fills a real gap -- the current quickstart page is just a "work in progress" stub.

No blocking issues. Two minor suggestions below.

Suggestions

1. Missing import torch in Custom Quantized Layers example -- The last code example uses torch.bfloat16 but does not import torch. Add import torch at the top of that snippet.

2. Missing import torch in QLoRA example -- The BitsAndBytesConfig snippet under "4-bit Quantization" also omits import torch while using torch.bfloat16.

3. Consider nf4 over fp4 in examples -- The 4-bit quantization example does not set bnb_4bit_quant_type. The existing integrations docs and most real-world QLoRA usage default to nf4 for better accuracy. Consider adding bnb_4bit_quant_type="nf4" to match common practice and the QLoRA paper's recommendation.

4. Commit messages are generic -- The two commits are titled "documentation" and "doc". Not blocking, but squash-merging with a descriptive message like "docs: add quickstart guide with usage examples" would improve the git history.

Cross-PR Conflicts

• PR #1734 ([docs] Improve Quickstart with examples and fix 'What's next?' links) by @Dhiraj309: Direct file-level conflict on docs/source/quickstart.mdx. Both PRs replace the same placeholder content with different quickstart guides. Only one can merge cleanly; the other will need to rebase. A maintainer should decide which approach to adopt, or ask the authors to coordinate. PR #1734 has been open since August 2025 with no reviews.

Checklist

• Security: Clear (docs-only change, no code execution)
• Downstream impact: None
• Tests: Not applicable (docs-only)
• CI: Not triggered (fork PR -- a maintainer needs to approve the workflow run before merge)
• API accuracy: Code examples reference correct class names and constructor parameters per the actual source (Linear8bitLt, Linear4bit, Adam8bit). Cross-checked against api_surface.md and source in bitsandbytes/nn/modules.py.

[github-actions]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[matthewdouglas]

Looks good, thank you! Just a couple minor nits :)