Heartwood's CommonMark-superset content format gets a proper name
(Heartmark) so projects outside Heartwood can describe what they
support. The reference implementation, formerly Heartwood.Content,
becomes Heartwood.Heartmark — same convention as cmark for CommonMark
or Markdig for Markdown.

File renames:
  CONTENT-FORMAT.md -> HEARTMARK.md
  decisions/024-extended-markdown-content-format.md -> decisions/024-heartmark.md

Wire-format field content_format_version renamed to heartmark_version
across PROTOCOL.md.