Adapt Aurora's Magnitude docs for mp-units

The major missing work here is to see how the latex math, inside dollar
signs, translates to the docs.

I also added a new constant, `pi`, to make the docs correct: this should
be much more user friendly.

This is not intended to be "complete".  Rather, the goal is to deliver
the raw content so that experts can massage it.

src/core/include/units/magnitude.h

@@ -398,6 +398,11 @@ struct pi_base {
   static constexpr long double value = std::numbers::pi_v<long double>;
 };
 
+/**
+ * @brief  A convenient Magnitude constant for pi, which we can manipulate like a regular number.
+ */
+inline constexpr Magnitude auto pi = magnitude<base_power<pi_base>{}>{};
+
 ////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
 // Magnitude equality implementation.