Document the OciDeck on-disk format: Marp-compatible Markdown, project
folder layout, YAML front matter (incl. TLP and base64url style profile),
per-slide-type markup, image caption sidecars, special HTML comments, and
the portable .ocideck package. Link it from the README.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
