[Doc] Add user guide of speculative decoding

[zhaomingyu13]

What this PR does / why we need it?

Add user guide of speculative decoding that includes n-grams, EAGLE, and MTP.

Does this PR introduce any user-facing change?

No

How was this patch tested?

• vLLM version: v0.12.0
• vLLM main: vllm-project/vllm@ad32e3e

[gemini-code-assist]

Code Review

This pull request adds a new user guide for speculative decoding in vLLM Ascend. The guide covers n-grams, EAGLE, and MTP methods with code examples. The content is very helpful, but I've identified a few areas where the text can be improved for clarity and readability. My comments focus on fixing grammatical errors, run-on sentences, and formatting issues to enhance the overall quality of the documentation.

[github-actions]

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[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.