[docs] Community methods

[stevhliu]

Updates the Community methods section:

• remove Token merging, DeepCache, and TGATE as these haven't been updated in a while and mostly support older UNet models. Diffusers' built-in caching covers some of these same needs
• remove xFormers because this doc was quite thin (pip install and a link to set_attention_backend) and inbound links to it are redirected to the Attention backends doc
• FreeU moved to Inference > Using diffusion pipelines because Diffusers has an enable_freeu method so its no longer a community method, and it changes how images look rather than how fast they're made
• greatly simplify CacheDiT doc for Diffusers users who probably won't need to use BlockAdapter or PatchFunctor. these are more for users adding new models to CacheDiT, so a link to it will suffice on our end. likewise, also simplified the sections that went in-depth on the design of DBCache and TaylorSeer

[HuggingFaceDocBuilderDev]

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[sayakpaul]

Great simplification!