TypelevelSiteSettings - deprecate top level color properties in favor of a nested object

[jenshalm]

Previously the color properties were all individual top level properties, drowning out the most relevant API elements. Secondly, their naming reflected the actual color, not their usage, which means that any changes in look & feel would lead to the need for new deprecations to avoid misleading names.

This PR moves all top level color properties into a new nested object colors and renames them so that they reflect their usage. The existing properties are all deprecated.

A possible change I would recommend, but did not do in this first version of the PR in case you want to be very conservative about it: I checked within the Typelevel organization, and not a single color property is used anywhere. So there is an option to remove these properties right away. In case you'd prefer that, too, I'll update the PR.