md: experimental `saref-dev md`, the TS as md-specs-dev Markdown

Writes target/md/TS-<number>.md (YAML front matter, ::include lines),
clauses/*.md and clauses/media/*, in the Markdown flavour of the ETSI CTI
toolchain (forge.etsi.org/rep/cti/md-specs-dev): numbered ATX headings,
`> NOTE:` blockquotes, `<span id="_ref_i.n">` reference entries and
[\[i.n\]](#_ref_i.n) citations, `ABBR        expansion` entries, bold
figure and table captions, annexes `# Annex A (normative): …`, History
table. Uses documentation/ts.yaml like `saref-dev ts`.

The ontology annex reuses TSGenerator.describe_* with the Word
primitives redirected to Markdown.

Tested on saref4inma with the ETSI Docker images (processMDSpec,
pandocFilter, pandoc, update_format_styles): Word document with figures,
tables, NOTE and EW styles.