docs: document the construct vocabulary and the first-class division decision

A Fidelity constructs section covering the six kinds, what each carries, and which
producer constructs land on it, plus a worked package showing a tracked insertion inside a
content control and a footnote marker whose body lives in the definitions table.

The four bounds are recorded as decisions rather than omissions: extents are block-scoped
and run-level hyperlinks stay on ContentRun.hyperlink; there are two group variants, one
per block flow; division is first-class rather than degraded to a content control; and the
quarantined residue channel is deliberately not part of this vocabulary.

The division decision is spelled out with its reasoning.
ContentSection cannot host an ODF text:section -- that is page geometry, one pageSize and
margins pair, and it does not nest, while a division nests arbitrarily and usually changes
no page geometry at all -- and burying a structural container in the form-control
vocabulary would make contentControl mean two unrelated things.
It clears the no-format-specific-kinds bar on a real analogue, tagged PDF's /Sect and /Div,
and is spelled division because section already names the page-geometry container
descriptor.

The definitions-tables section gains the three construct tables and why each is its own
root field rather than another tenant of definitions.

Refs #22, #24

Author: Mearman