[docs] Improve navigation

[stevhliu]

Reorganizes the navigation around user intent more, at a high-level:

Get started
Modular Diffusers
Inference
Optimize and scale
Train and fine-tune
Resources
Contribute
API

• Group pipeline, adapter, and prompting content together under Inference
• Group performance, optimization, quantization, hardware acceleration, and serving under Optimize and scale
  • Refactor the Basic performance doc as an overview
• Rename and regroup training subsections into Model recipes and Training methods
• Split Resources from Contribute for better discovery

[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.