[docs] Community pipelines

[stevhliu]

This PR combines the "Community pipelines", "Load community pipelines and components", and "Contribute a community pipeline" docs together to make it easier to find everything about community pipelines on one page. Previously, it was a bit spread out and there wasn't really a clear benefit to it.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.