docs: add overview and common cases

[ernado]

• [x] Add quick start section
• [x] Describe "Convenient errors" concept
• [ ] Describe request/response types generation and how ogen handles multiple variants
  • [ ] Request
  • [x] Responses
• [ ] Describe supported formats and types
  • [x] Optional
  • [x] Map
  • [x] Primitives
  • [x] Sum types
  • [ ] Enums
  • [x] Any
• [x] Describe type discriminator inference algorithm in general
• [x] Add FAQ/Troubleshooting section
  • [ ] Define when and why ogen uses jx.Raw
  • [ ] Describe ErrNotImplemented
  • [ ] Describe supported content types and io.Reader fallback
• [ ] Add note about ogent