Use YAML in Markdown Frontmatter

In the context of the Lexical.cloud cloud catalog,
facing what format to capture metadata in markdown frontmatter for Hugo,

we decided to store metadata in the frontmatter of markdown as YAML
and against using Hugo’s alternatives of JSON or TOML

to achieve a simple format for metadata curation
and compatiblity with other static site generators,
accepting that consistent whitespace will need enforcement.