The section definitions were kind of all over the place, but have slowly

been progressing toward a more organized set of definitions on the types

of sections, the rules around headers, and common options.
This change improves this considerably. The "Section and Header Formats"

page is now "Section Definitions" (name may still be in flux), which

starts by providing an overview of sections, then better defines the

format for section headers, and then goes into full detail on the types

of sections.
Some of that full detail was already there, but is now cleaned up. The

Metadata and Preamble content sections are now better defined. Common

options are now listed separately for container and content sections

here in one place.
This simplifies the Section Hierarchy page (which was formerly "DiffX

Sections"). The preamble and metadata sections can now heavily rely upon

the standard definitions, instead of having to repeat things.

Built the docs and checked through to make sure the links worked, the

text was correct (though this needs a second pair of eyes), and that

there were no new spelling errors.