docs: complete observability CLI reference

[gregggreg]

Summary

• remove the stale top-level nightshift snapshot entry and document nightshift budget snapshot
• add usage examples and complete flag/default tables for status, logs, stats, report, busfactor, doctor, and budget snapshot
• document supported periods, providers, timestamp formats, output modes, aliases, and relevant flag interactions from the Cobra implementation

Impact

The CLI reference now matches the implemented observability and diagnostic command surface, making these commands usable without consulting generated help or source code.

Validation

• npm ci in website
• npm run build in website
• go test ./...
• generated --help checks for all documented commands
• confirmed nightshift snapshot is not a command
• git diff --check