"getting started" documentation is misleading

The Getting Started documentation presents the creation of new units as if new quantities were being created. Even the text in the exception raised in the example speaks of quantities.

Specifically, the following examples need to be changed to be less misleading:

> ```
> from scimath.units.length import foot, inch, meter
> 3 * foot
> 0.9144000000000001*m
> foot / inch
> 12.0
> foot / meter
> 0.3048
> ```
>
> However, adding incompatible units raises an IncompatibleUnits exception:
>
> ```
> from scimath.units.electromagnetism import volt
> from scimath.units.mass import kilogram
> 1 * volt + 2 * kilogram
> Traceback (most recent call last):
> File "", line 1, in
> File "scimath/units/unit.py", line 62, in add
> raise IncompatibleUnits("add", self, other)
> scimath.units.unit.IncompatibleUnits: Cannot add quanitites with units of 'm**2*kg*s**-3*A**-1' and 'kg'
> ```