[WIP] [Docs] Add docs for EP (expert parallelism)

[JustinTong0323]

TODO:

1. Add introduction for EP
2. Explain how tp_size, ep_size and moe_ep_size are calculated and maybe keep in mind the way to calculate is not same as vllm
3. Add several examples, including usage for eplb
4. Maybe simplify the doc, it is too wordy now.
5. ..

Motivation

Modifications

Accuracy Tests

Benchmarking and Profiling

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.
• Work with maintainers to merge your PR. See the PR Merge Process

[gemini-code-assist]

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