fix incorrect links in documentation

[rchen19]

Motivation

Some links on current Documenation contains links that are actually file path in the repository, e.g., https://sglang.readthedocs.io/en/latest/backend.html#docs/en/custom_chat_template.md, which should be pointing to the file docs/en/custom_chat_template.md in the repo, but on the website, clicking on the link will not take you anywhere.

Modifications

Change the links that contain file paths to full URL pointing to github readthedocs pages, e.g., change docs/en/custom_chat_template.md to https://github.com/sgl-project/sglang/blob/main/docs/en/custom_chat_template.md

`here <https://sglang.readthedocs.io/en/latest/custom_chat_template.html>`

Checklist

• Format your code according to the Contributor Guide. No code changes
• Add unit tests as outlined in the Contributor Guide. No need for tests since there is no code change
• Update documentation as needed, including docstrings or example tutorials. This PR is purely about documentation

[zhyncs]

@rchen19 Thanks for your contribution!