docs(#14): add comprehensive API reference documentation

[dennismdejong]

Fixes #14

Adds a comprehensive API reference document (docs/api-documentation.md) documenting all known Petlibro API endpoints, their request/response formats, and field descriptions.

Covered endpoints:

• Authentication (/member/auth/login)
• Device management (list, baseInfo, realInfo, dataRealInfo, attributeSettings)
• Feeding plans (dry + wet)
• Fountain water modes and drinking data
• Litter box commands and configuration
• Events and work records
• Pet management and RFID collar data
• All control/setting endpoints
• Member/account info

All documentation is based on reverse-engineering the code in api.py and the device classes.