Docs: Fix section for Antora: "System Configuration and Customization"

Docs: Fix section for Antora: "System Configuration and Customization"

This commit does the known tasks to prepare each main docs section for
Antora:

  - Add chapter labels to nav file items
  - To match existing docs "chapter" level headings, combine adoc files as
    needed so that each "chapter" from the old docs structure has a single
    file linked from the nav
      - In this case, the only change was in advanced_features.adoc,
        where two sections had the top heading level (seems like a mistake).
        I bumped the second one down ("Binary MARC21 Feeds") so it is a
        subsection of the file. Ironically, since this commit also bumps
        up all headings by one level, it means that heading is the only one
        not changed in this commit.
  - Upgrade all heading levels in each adoc file so that the top-most
    heading is a "Level 1" heading (example: "= Chapter Name =")
  - Add ":toc:" below top heading of each file

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