Document role gradient colors

[DA-344]

Adds documentation for the role's gradient colors

[Lulalaby]

I might be wrong here, but it seems like the holographic style needs the following config:
primary (11127295), secondary (16759788), tertiary (16761760)
And tertiary can't be set for normal gradients.

I haven't tested around enough, but that was my impression

[advaith1]

I assumed that was a client only restriction, is it enforced server side?