[Docs] add quantization docs

[yinfan98]

Motivation

Add docs for quantization. This PR is Change from previous one : #2572. cc: @zhaochenyang20

Modifications

This PR adds documentation for enabling online quantization and offline quantization using SGLang.
The modifications can be summarized as follows:

Add document of quantization docs/backend/quantization.md
Modified docs/index.rst, to inlcude the quantization docs into SGLang documentation.

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.

[zhaochenyang20]

I personally think we should only keep the markdown file, since quantization ipynb would relaunch the kernel multiple times and this is not a basic feature. @shuaills

[zhaochenyang20]

@yinfan98 Fix lint and remove ipynb. thansk