refactor: pi mag_constant refactored to be one-liner

docs/users_guide/framework_basics/systems_of_units.md

@@ -184,10 +184,9 @@ For some units, a magnitude might also be irrational. The best example here is a
 is defined using a floating-point magnitude having a factor of the number π (Pi):
 
 ```cpp
-struct pi : mag_constant {
+inline constexpr struct pi : mag_constant {
   static constexpr auto value = std::numbers::pi_v<long double>;
-};
-inline constexpr pi pi;
+} pi;
 ```
 
 ```cpp

src/core/include/mp-units/framework/magnitude.h

@@ -584,10 +584,9 @@ constexpr Magnitude auto mag_power = pow<Pow>(mag<Base>);
 /**
  * @brief  A convenient Magnitude constant for pi, which we can manipulate like a regular number.
  */
-struct pi : mag_constant {
+inline constexpr struct pi : mag_constant {
   static constexpr auto value = std::numbers::pi_v<long double>;
-};
-inline constexpr pi pi;
+} pi;
 
 [[deprecated("Use `mag<pi>` instead")]] inline constexpr Magnitude auto mag_pi = mag<pi>;