Docs: Convert subnav files to included partials

Docs: Convert subnav files to included partials

This commit adds labels to the Cataloging nav, and combines files from
subnav items that happen to have their own adoc files (but don't really
belong in the nav). This also bumps heading levels so that each file
starts with a "Level 1" (book title) heading, which Antora wants. And we
add a Table of Contents to the top of each file.

Signed-off-by: Remington Steed <rjs7@calvin.edu>