Create Best Practices Docs

Some concepts are non-intuitive. Add How-To or best practices for following topic:

- Distinction Quantity vs. Unit
- Create new units
- linear units (eg. metre -> feet)
- with focus on combined quantities (velocity)
- better not define `FooLengthPerBarTime` velocity unit
- but define both a `FooLength` length unit and a `BarTime` time unit, then use "Per" method to combine them
- Using variables to capture units
- Point out the existence of the Units nuget at the point where I describe creation of new units.

Start by locating the repository's existing documentation for quantities and units. Add guidance covering the Quantity-versus-Unit distinction, creating linear and combined units with the "Per" method, capturing units in variables, and where to find the Units NuGet package; done means each requested topic is documented.

Written by the indexing model from the issue text.