Add documentation links to graphviz colors

[jsoconno]

Is your feature request related to a problem? Please describe.
No. It is to make using some features of the library more clear in our documentation.

Describe the solution you'd like
Add links to the Graphviz documentation on colors so that users know what color options are available. Here is a link to the website: https://graphviz.org/doc/info/colors.html.

Describe alternatives you've considered
We could explicitly add them as validation options in the settings classes, but there are just too many options to choose from so it is more practical to add a link to the documentation to aid users.

Additional context
This is only to help users better find some of the formatting options associated with the Graphviz engine under the hood.