docs: Add more cross-linking to Brand docs. #4907

This has come up in recent cross-team conversations: we link to Brand docs in a place or two on pf.org, but there are more places we can do this better to ensure that Red Hat users have easy access to essential info. For example, colors, icons, typography, etc. This will likely be especially relevant as we worked towards a unified theme option.

Jira Issue: PF-3436