ts: optional documentation/ts.yaml for document metadata, clauses and annexes

- document: TS number/part, version, date, titles, work item, keywords,
  overriding the ETSI work item or the draft placeholders
- clauses: ordered documentation sources with the headings to add when
  a source has none; any extra source becomes a clause
- annexes: documentation sources (normative or informative) and the
  ontology reference (title, normativity, sections), in any order
- defaults unchanged when the file is absent