docs: restructure README with Data Modes, Entity Types sections

- Add Data Modes section explaining Entities vs JSON Attribute mode
- Add Entity Types section with Sensors, Lights, Climate details
- Document smart temp_offset behavior for climate entities
- Document HVAC color behavior (heating/cooling override, idle uses gradient)
- Document light behavior (color.low for off lights)
- Add Title & Header guide and Styling guide sections
- Add Contributing section

Author: Ondrej Machala