Add more model documentation

Create documentation outlining the model decisions made in the last few months:
- ASHP
- Alarm selection
- Energy meters vs heat meters
- Generators
- ATS
- Fan speed modes (TWENTY_PERCENT, etc.)

Start by reviewing the repository’s existing documentation and the history behind the listed model decisions. Document the decisions for ASHP, alarm selection, energy versus heat meters, generators, ATS, and fan speed modes, using terminology consistent with the project. Done means all six topics are covered in a discoverable documentation page.

Written by the indexing model from the issue text.