Docs: add GCWU theme documentation

Question about these docs @pjackson28:

1. Would you like me to change the the 3.1 content links in the [Content page-specific instructions (cont*.html)](https://github.com/wet-boew/wet-boew/wiki/GC-Web-Usability-Theme#content-page-specific-instructions-conthtml) section to application template links?  Currently they're removed in this PR since the cont*.html templates don't exist in 3.0.