Markdown glitch in facts.markdown
Fix inaccurate description of = operator (which is not only string equality)

After review from Nick and Chris.
Change query.markdown to query structure overview; move its content to ../tutorial.markdown
Adjust headers and add intro paragraph to tutorial.markdown
Update puppetdb 1.1 nav snippet with broken nested menus
De-nest the nav menus

This makes it a little less logically rigorous, structure-wise, but it
ends up looking less broken, and I don't think it's ultimately going to
be confusing.
Add API index page w/ summary of what the API consists of; still contains TODOs.
Add links to the API landing page
Fix broken links in curl.markdown; minor clarifications in commands.markdown