[docs] Files and formats

[stevhliu]

This PR attempts to clarify the difference between file types (safetensors, ckpt, bin) and storage layout (multifolder, single file) to avoid confusion when discussing "Diffusers" format and "single file" format. The most common misconception is that safetensors is just a file type, and single file or multifolder layout can both hold safetensors files. The PR also removes the old "Load safetensors" doc and discusses it in the current one here where it is more contextually relevant.

todo: ~- [ ] add Keras section once Space is fixed~

[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.

[stevhliu]

We can add the Keras section back whenever @sayakpaul gets a chance to fix it (no rush!), and I think it'd also be helpful to move the content from here (usage examples for from_single_file in the API docs) to this doc in a separate PR. WDYT?

[sayakpaul]

We can add the Keras section back whenever @sayakpaul gets a chance to fix it (no rush!),

Yeah sure. Actually, the problem is with the installation and the code itself.