Allow unnumbered clauses

[opoudjis]

Clauses in the preface are usually unnumbered. Clauses in the main section are almost always numbered, and we would discourage the alternative, since referencing clauses is so critical in standards. So we have not implemented unnumbered clauses, although we have implemented unnumbered blocks.

But there are edge cases for unnumbered clauses, and we should support them even if we don't encourage them. One such arises in https://github.com/metanorma/iso-10303-srl/issues/41, with bare documents. (Bare documents are HTML renderings of content explicitly intended to be standalone pieces of HTML and not standards documents—which is why we don't want their clauses numbered either. In this case, they render schemas on their own, without their accompanying definitions.)

Unnumbered clauses may still be cross-referenced, and if they are, they need to be referenced as with unnumbered preference clauses, by title. In fact, we have done that in some flavours already. They also need to be skipped in numbering, although that's already been taken care of in implementation.

[opoudjis]

@ronaldtse asks in metanorma/isodoc#574

I have a question. For bare, why do we need a fake title and then give it a section for the schemas? Why not just use the title?

Asciidoctor (and Metanorma) is so opinionated about setting titles, that if we don't set a fake section + title, the document will end up as a foreword, and by default "Foreword" will overwrite the supplied title anyway. In fact, there is an easier workaround than this:

[preface]
== {{ title }}

Preface clause titles aren't numbered in ISO, after all.

it's just that I just can see this coming back repeatedly, and it has already on occasion, so I thought we might as well do this now.