Doc: add DiffusionGemma to supported models

[BPbruce]

Add DiffusionGemma to the supported models table.

DiffusionGemmaForBlockDiffusion is already registered in the model registry and has a test registry entry for google/diffusiongemma-26B-A4B-it.

This follows the model support added in #45163.

[github-actions]

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[mergify]

Documentation preview: https://vllm--46466.org.readthedocs.build/en/46466/