Add Doxygen documentation for R(θ, φ) gate matrix implementation

[Copilot]

Addresses review feedback on PR #1283 requesting documentation for the rMat function implementation.

Changes

• Added proper Doxygen-style function docstring with @brief, @param, @return, and @details sections
• Documented the mathematical formula: R(θ, φ) = exp(-i*θ/2*(cos(φ)X + sin(φ)Y))
• Documented the explicit matrix representation: [[cos(θ/2), -i*e^(-iφ)*sin(θ/2)], [-i*e^(iφ)*sin(θ/2), cos(θ/2)]]
• Added parameter descriptions for theta (rotation angle) and phi (rotation axis angle)

The documentation clarifies the relationship between the exponential form and the matrix elements computed in the implementation, following the standard Doxygen format used throughout the codebase.

/**
 * @brief Computes the matrix representation of the R(θ, φ) gate.
 * @param theta The rotation angle θ.
 * @param phi The rotation axis angle φ.
 * @return The gate matrix for the R(θ, φ) rotation.
 *
 * @details The R(θ, φ) gate is defined as R(θ, φ) = exp(-i*θ/2*(cos(φ)X + sin(φ)Y)),
 * which results in the matrix:
 * [[cos(θ/2), -i*e^(-iφ)*sin(θ/2)],
 *  [-i*e^(iφ)*sin(θ/2), cos(θ/2)]]
 */
GateMatrix rMat(const fp theta, const fp phi) {
  // ... implementation
}

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[burgholzer]

@copilot turn this into a proper doxygen-style function docstring, with a @brief, a @details section, and the parameter descriptions.

[Copilot]

@copilot turn this into a proper doxygen-style function docstring, with a @brief, a @details section, and the parameter descriptions.

Converted to proper Doxygen-style documentation with @brief, @param, @return, and @details sections in commit fb137fc.

[burgholzer]

pre-commit.ci autofix