Add docs on conversion factors

[zainab-ali]

The "Roll Your Own" section of the guide doesn't explain how to convert from one unit to another.

This logic is a bit tricky, especially until #50 is properly solved, but would still be worth documenting

Where to Start

Take a look at the AngleImplicits for a good example on how to use libra's ConversionFactor.
Add an example of converting from one unit to another (e.g. Roman feet to thumbs) to the "Roll Your Own" section of the docs. It might also be worth documenting that the number of conversion factors grows exponentially with each new unit, and linking #50.