[Docs] Add quantization docs

[Edenzzzz]

Motivation

Re-opens #3253 with reviews addressed.

Modifications

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
• For reviewers: If you haven't made any contributions to this PR and are only assisting with merging the main branch, please remove yourself as a co-author when merging the PR.

[Edenzzzz]

cc @zhaochenyang20

[Edenzzzz]

I think this means online quantization? Loading offline awq weights is already supported.

[zhaochenyang20]

Thanks. I will give credit to you, james and fan.

[zhaochenyang20]

We should move it to reference and add it in index.rst.

[Edenzzzz]

@zhaochenyang20 Added

[zhyncs]

FYI In the upcoming release, we will default to using sgl-kernel's W8A8 Int8 and FP8 instead of vLLM's W8A8. We have achieved best performance across on all sm80, sm89 and sm90.

[zhaochenyang20]

Great. Wait, we need to change this a bit