Add Doxygen documentation for R(θ, φ) gate matrix implementation (#1284)

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.

```cpp
/**
 * @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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Author: 3 people

src/dd/GateMatrixDefinitions.cpp

@@ -65,6 +65,17 @@ GateMatrix rzMat(const fp lambda) {
                      {std::cos(lambda / 2.), std::sin(lambda / 2.)}}};
 }
 
+/**
+ * @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) {
   const auto cosTheta = std::cos(theta / 2.);
   const auto sinTheta = std::sin(theta / 2.);