[docs] Custom community components

[stevhliu]

Follows up on #5620 with a few suggestions and improvements 🙂

1. Start by introducing and clarifying what community components are instead of discussing the more technical aspects of it (need to add Python modules for them).
2. Gradually reveal to the user they're loading a community UNet and pipeline.
3. Clarify a bit more that the users themselves aren't implementing the UNet, and they're just loading a custom one that we've already provided (same with the pipeline)

[sayakpaul]

Thank you! Just some minor nits.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.