feat(Iconography): Adds guidance on color inheritance

From @jtucker_gl :

I've seen lots of discussion in issues, MRs, and Figma over the last few weeks about whether icons should inherit the color of their parent or if they should be styled independently of text color. I looked over https://design.gitlab.com/product-foundations/iconography this morning and found that we don't have any guidance on this. Rather than pushing through what's effectively a new design system constraint through our dark mode rollout, let's discuss this explicitly and make a decision on whether this is the right path for the GitLab product UI. I'm not creating any more "update icon color" issues until we complete this discussion, and I will not consider these issues a blocker to transitioning dark mode to beta.

I've opened this MR to start and house that discussion and document the result.