[Feature] Add Tutorial for Constraint Decoding

[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

A better document for constraint decoding.

Related resources

https://sgl-project.github.io/backend/openai_api_completions.html#Structured-decoding-(JSON,-Regex)