[docs] Document BTP service key authentication usage #75
Description
Update documentation so users can:
- See a full example of using service key authentication with ADT CLI via env vars, CLI flag, and adt.config.ts
- Understand service key JSON structure and where to acquire it
- Find troubleshooting steps for errors
- Copy-paste the relevant commands
Files: packages/adt-cli/README.md, packages/adt-auth/README.md
Parent Epic: #70
Blocks: #cli-auth-selection