Make settings configuration more discoverable

[choldgraf]

trafficstars

This tries to improve the discoverability and readability of some project- and page-configuration sections. It does a few things:

• Creates a new top-level section for project-wide configuration and setup. Moves some content there from Authoring so Authoring isn't quite so huge.
• Moves some more content from Reference into this section, so that it's more discoverable.
• Breaks up the really long Frontmatter docs into an overview-style page and a reference-style page.