[docs] Using diffusion pipelines

[stevhliu]

Updates the Using diffusion pipelines section:

• reorder the guides so they match how Diffusers is used (loading and using pipelines/models → choose format → schedulers → prompting → reproducibility → customize behavior with callbacks → share)
• better docs on using from_pipe to switch tasks
• added a section for GGUF file formats
• added a section on classic CFG vs true_cfg_scale/distilled guidance for classic and modern pipelines
• better clarification where appropriate for standard pipelines vs modular pipelines
• fix some code snippets that would've loaded the wrong class or misuse device_map and dtype, pick clearer modern defaults, more consistent voice and prose

[HuggingFaceDocBuilderDev]

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[sayakpaul]

Very nice!

[stevhliu]

a question is whether its worth keeping some of these sections below that's related to the older checkpoints. maybe @asomoza can also weigh in here on whether or not these checkpoints, like stable diffusion, still get much usage

• safety checker
• rescaling schedules
• prompt weighting, i guess it works for Flux as well but it doesn't necessarily help that much?
• pipeline callbacks like for SDXL. should we encourage users to use Modular Diffusers instead for this behavior?

[sayakpaul]

a question is whether its worth keeping some of these sections below that's related to the older checkpoints

I think we could still keep them since these older techniques still seem to be used in the community, particularly callbacks. Maybe we could group the others and move to a section called "Older techniques" and keep it collapsed?