Allow interps to contain headers that don't reference anything

Reg Z, for example, has an introductory section which isn't related to any regtext/appendices.

Also:
* Allows interp paragraphs without a marker (as occurs in that same section).
* Uses node_text, in case the header contains markup
* Replaces an existing, duplicate test (There were two "test_build_supplement_tree_repeats"s).