refactor: 💥 mag_constant now takes a symbol and a value and the class deriving from it must be final

docs/users_guide/framework_basics/systems_of_units.md

@@ -184,9 +184,7 @@ 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
-inline constexpr struct pi : mag_constant {
-  static constexpr auto value = std::numbers::pi_v<long double>;
-} pi;
+inline constexpr struct pi final : mag_constant<symbol_text{u8"𝜋", "pi"}, std::numbers::pi_v<long double>> {} pi;
 ```
 
 ```cpp