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.

docs/framework.rst

@@ -17,6 +17,7 @@ Framework Basics
     framework/quantity_points
     framework/quantity_kinds
     framework/dimensions
+    framework/magnitudes
     framework/units
     framework/arithmetics
     framework/constants