Markdown types

[davesnx]

The markdown backend was emitting fenced code blocks without a language identifier (bare ` ``) for OCaml API declarations (type signatures, value definitions, module types, etc.), while other backends correctly assumed OCaml as the default language.

• Changed the documentedSrc function in src/markdown2/generator.ml to emit `ocaml `` for these code blocks, aligning with the behavior of the HTML and other backends.
• Refactored documentedSrc to collect Code, Documented, and Nested items into a single unified code block with inline doc comments, rather than rendering them as separate blocks.
• Verbatim ({v ... v}) and Math code blocks are left without a language tag, as they are not OCaml code.
• Explicit language tags on Source blocks (e.g. {@python[...]}) continue to be preserved as-is