docs: update vLLM-Omni Cosmos3 compatibility status

[david6666666]

The "Generator with vLLM-Omni" section has some stale references now that the upstream PRs have landed.

Compatibility status paragraph

• #3454 (text-to-image / text-to-video / image-to-video) is merged on vllm-omni main, not "being upstreamed."
• video-with-sound (Add Cosmos3 sound generation vllm-project/vllm-omni#4073) is also merged — it's listed as a pending follow-up.
• Action is still in review (Add Cosmos3 action modality vllm-project/vllm-omni#4102); video-to-video is still planned.
• Linked the maintained recipes (Cosmos3-Nano / Cosmos3-Super) for per-modality usage.

Install snippet

• Since #3454/#4073 are merged, install from main instead of the @refs/pull/3454/head PR ref.

Dead link

• examples/online_serving/cosmos3 404s (it doesn't exist on main, and isn't added by #4102). Repointed both references to the maintained recipes/nvidia.

Scoped to the vLLM-Omni section; Docker quick-start and parallelism options left as-is.

[david6666666]

Note on the status line: action (vllm-project/vllm-omni#4102) is still in review here intentionally — the backward PR will be issued and land soon, and I'll flip it from "in review" to "merged" once it does.

[lishunyang12]

Note on the status line: action (vllm-project/vllm-omni#4102) is still in review here intentionally — the backward PR will be issued and land soon, and I'll flip it from "in review" to "merged" once it does.

Action generation pr will be integrated soon and @bastefaniak will land inverse kinematic in the next pr. The recipe will be maintained here https://github.com/vllm-project/vllm-omni/tree/main/recipes/cosmos.