Fixes #12190

The colors documentation did not explain how to customize axis tick labels,
axis title colors, or grid line colors. This made it difficult for users
on dark backgrounds to know how to change these.

Added a new "Axis label and tick colors" section covering:

• ticks.color for tick label colors per scale
• title.color for axis title colors
• grid.color for grid line colors
• Chart.defaults.color for global overrides