[Docs] Add community projects section to docs

[DN6]

What does this PR do?

We have a lot of great community projects built with Diffusers. Propose adding a community projects section to highlight them.

I put this in it's own section for now, but I'm happy to move it to a more appropriate place if needed.

Fixes # (issue)

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[stevhliu]

Very nice, thanks! Just need to align the title in the toctree with local and then the test should pass :)

[HuggingFaceDocBuilderDev]

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