3.0.0

Add Sphinx documentation

- docs/conf.py: furo theme, autodoc + napoleon + viewcode + intersphinx
- docs/index.rst, quickstart.rst: project overview and usage examples
- docs/api/: API reference for EcosizerEngine, Building/ClimateZone,
  DHW systems, equipment components, and SimulationRun
- docs/Makefile + make.bat: `make html` builds to docs/_build/html
- pyproject.toml: add [docs] optional-dependencies (sphinx, furo)
- Minor docstring fixes in MultiPassRTPSystem and SimulationRun
  to resolve Sphinx/docutils parse warnings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>