Issue #3619191: Document the three classes a structure builder falls into

Structure builders are the bulk of what this module and the modules built on it do, and there was nothing telling an author where a new one belongs.

Counting them across the forks gives 77, and every one is either the shape of a field, a bundle rendered as a nested item, or something computed. The class decides where the code belongs: the first goes to a structure service of this module, the second is a field map, the third stays in a module of the site behind an extension point.

Two numbers from the same count are worth stating next to the classes: 24 of the 77 exist in more than one fork and only 6 of those are identical, so a builder moved into a shared place has to answer with the union of the keys, or the site that had the other version loses keys its front end reads.

Documentation only; no behaviour changes.

Closes #3619191