Documentation: Clarify best practices surrounding color palette names

[aduth]

Related:

• https://core.trac.wordpress.org/ticket/46698
• New color picker improvements, second round #2699
• Implemented mechanism to use classes for configured colors. #5273

Current documentation: https://github.com/WordPress/gutenberg/blob/master/docs/designers-developers/developers/themes/theme-support.md

The editor-color-palette theme supports enable a theme developer to specify a custom color palette for their themes. Each color should include a name, slug, and color values.

The current documentation neither describes each value, nor importantly does it set clear guidelines around the purpose a name should fulfill in providing human-friendly (and accessible) labels describing the color.

Per the related ticket, it may be good to include guidance around dynamic values (such as those sourced from a site option). For example, while not all dynamic values could be described programmatically, it may be advisable to provide meaningful names for at least the default values when applicable.

[nickylimjj]

Am picking this one up. Will submit a PR by Apr 18!