refactor: reorganize documentation - README as quick overview, detailed docs in mdBook

- Simplify README to be a concise quick-start guide
- Move comprehensive async operations documentation to mdBook
- Create dedicated Features section in book with chapters for:
  - Async Operations overview
  - Database Operations (regular, fixed, Active-Active)
  - Subscription Management
  - Network Connectivity (VPC, PSC, TGW)
  - ACL Management (rules, roles, users)
  - User & Account Management
  - Output Formats and filtering
  - Profile Management
- Update book SUMMARY.md with new structure
- Remove FEATURES.md as content now in book
- Better separation between quick reference (README) and detailed docs (book)

Author: joshrotenberg