Revamp docs

[h4x3rotab]

This is a tracking issue of the general dstack revamp effort.

Progress

• README - Reviewed and complete
• Deployment guide (docs/deployment.md)
  • Quick Start section added
  • Advanced Features merged
  • Troubleshooting merged
  • User review pending
• Security docs (docs/security.md)
  • Best practices guide
  • Security audit reference added
  • Trust model (detailed)
  • User review pending
• Confidential AI use cases
  • inference.md created
  • agents.md created
  • training.md created
  • Consistency review pending
• Usage guide (docs/usage.md)
  • Content migrated from setup.md
  • User review pending
• SDK READMEs
  • Python
  • JavaScript
  • Rust
  • Go
• Code/cmdline testing

Goals

• Position as a Confidential AI framework
• User friendly deployment guide
• All cmdlines and code are tested