add code snippets showing common usage

[heckj]

The docs are roughed out, but would be notably improved with code snippets showing examples of using scales, and ticks, in action. Some potential paths for consideration:

• creating a scale for a relevant type (Int, Float, Double, potentially TimeInterval or Date)
• using a scale to map to a value for drawing purposes
• using a scale to get a list of tick values - automatic set
• using a scale to get range values for a preset array of domain values for Ticks
• using NiceNumber to get a "nice" minimum and maximum value for a range of values in order to build a scale with a "nice" domain

[heckj]

really hoping to wait until SwiftPM has full support inlined and available for @snippet references.