[Feature] Change contribution guide

[zhaochenyang20]

Checklist

• 1. If the issue you raised is not a feature but a question, please raise a discussion at https://github.com/sgl-project/sglang/discussions/new/choose Otherwise, it will be closed.
• 2. Please use English, otherwise it will be closed.

Motivation

https://sgl-project.github.io/references/contributor_guide.html

This has been outdated for long. We need to add guide on:

1. How to run docs CI, build it locally, compile it and clean the output and make PR.
2. How to do unit tests locally and add unit tests to CI.
3. How to write elegant unit test following other tests.
4. How to pre-commit.

Related resources

No response

[zhaochenyang20]

@shuaills This is a great issue to you 😂

[zhaochenyang20]

@shuaills Set up environments for beginner who even never use GPU.