WIP: User Guide + Examples

[shashi]

• [x] Content + Graphics
• [x] Layout (need to continue from docs/layout-guide.jl)
• [x] Interaction
• [x] Theme
• [ ] Dev docs
• [ ] Move REPL out into a new repo
• [ ] Compile to HTML

[randyzwitch]

With #158 merged in, do we go ahead and merge this in as well? At over a year old, current docs are just out-of-date for new users.