[DOCS] Colors tutorial #243

Is your feature request related to a problem? Please describe.

Colors can use a full tutorial, due to issues like #241

Describe the solution you'd like

Cover:

• Explicit colors: palettes, RGB hex
• Symbolic encodings: categorical, continuous, defaults
• Points vs edges
• Linked from main docs + pygraphistry homepage

Describe alternatives you've considered

Current docs (code, readme) are not enough