theme: document how menus on site are built and how theme is used

@carrythebanner and I had some trouble debugging a collision in page names that causes hugo to error out like this:

Error: Error building site: "/Users/jhetrick/Documents/z_etc/shift-docs/site/content/pages/pedalpalooza.md:1:1": duplicate menu entry with identifier "Pedalpalooza" in menu "main"

We were at a bit of a loss to figure out:

• what file was conflicting
• why it mattered (the same-named files were in different directories and shouldn't have conflicted).

What this issue requests is:

• an explanation of how the menu layout is created (we think it's largely based on frontmatter that exists in only SOME markdown files?). I'd suggest this belongs somewhere in the docs/ directory so people follow the intended pattern when creating new content. Maybe includes a "best practices for using/modifying this theme" too?
• some suggestions of how to debug problems like that. We never actually "solved it" - just worked around by renaming his page to something different.

Thanks for your help!