[Doc] refactor diffusion doc

[ZJY0516]

Purpose

refactor diffusion doc structure

cc @hsliuustc0106 @SamitHuang

before	now

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft.

BEFORE SUBMITTING, PLEASE READ https://github.com/vllm-project/vllm-omni/blob/main/CONTRIBUTING.md (anything written below this line will be removed by GitHub Actions)

[hsliuustc0106]

I think diffusion features should be under features dir:
features
diffusion->xxx

[ZJY0516]

I think diffusion features should be under features dir: features diffusion->xxx

The three-level menu doesn't look very clean

And what about some features both omni and diffusion model support? e.g., sleep mode

[hsliuustc0106]

I think diffusion features should be under features dir: features diffusion->xxx

The three-level menu doesn't look very clean

And what about some features both omni and diffusion model support? e.g., sleep mode
I suggest:

features
diffusion->xxx
omni-> xxx
sleep mode

[ZJY0516]

I think diffusion features should be under features dir: features diffusion->xxx

The three-level menu doesn't look very clean
And what about some features both omni and diffusion model support? e.g., sleep mode
I suggest:

features diffusion->xxx omni-> xxx sleep mode

diffusion features will be collapsed (nested) by default

[hsliuustc0106]

lgtm