[Feature] Rewrite the SRT Backend docs

[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

This doc has been outdated for a long time:

https://sgl-project.github.io/backend/backend.html#backend-sglang-runtime-srt

1. Only keep an explanation for server arguments and give the link to sampling parameters.
2. Add essential explanation for server arguments. Remember to add these kinds of arguments. https://lmsys.org/blog/2024-12-04-sglang-v0-4/#data-parallelism-attention-for-deepseek-models
3. A group of parameters have ##, ### is not allowed.
4. Use Models From ModelScope and Run Llama 3.1 405B move to reference, and potentially adds docs for deepseek.
5. change main readme.md.

Related resources

No such.

[simveit]

I can take this @zhaochenyang20

[zhaochenyang20]

@simveit Thanks so much!