Create folder for module guides, Beginner's Guide to Modules, pictures for BGTM

Create 'module_guides' subfolder within 'guides' folder. Add finalized
BGTM and linked images to 'module_guides' subfolder.
BGTM: Add yaml frontmatter and a sidebar
BGTM: Adjust header structure, replace code sample images, fix links
BGTM: Lock in sidebar for now: "Best practices for writing Puppet code"
Merge branch 'bgtm'