Add Doxygen documentation to header files

[Copilot]

Added comprehensive Doxygen-style documentation to Colour.h and Planet.h header files to improve code maintainability and API clarity.

Changes

Colour.h

• Class description for RGBA colour representation with normalized [0.0, 1.0] components
• Member variable documentation for red, green, blue, alpha fields
• Method documentation for constructors (float/unsigned short variants), operators, and all getters/setters

Planet.h

• Class descriptions for Planet and nested Orbit classes
• Member variable documentation for orbital parameters (aphelion, perihelion, period, inclination) and planet properties (name, radius, axial tilt, colour, satellites)
• Method documentation for all constructors, destructors, and accessor methods

Example

/**
 * @class Colour
 * @brief Represents a colour with red, green, blue, and alpha (transparency) components.
 *
 * Component values are internally stored as floats normalized to the range [0.0, 1.0].
 */
class Colour {
    float red;    ///< Red component, normalized to range [0.0, 1.0]
    float green;  ///< Green component, normalized to range [0.0, 1.0]
    // ...

Original prompt

Add documentation to the header files

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